diff --git a/Doc/c-api/bool.rst b/Doc/c-api/bool.rst
index ce8de6e22f4..c197d447e96 100644
--- a/Doc/c-api/bool.rst
+++ b/Doc/c-api/bool.rst
@@ -13,7 +13,8 @@ are available, however.
 
 .. c:function:: int PyBool_Check(PyObject *o)
 
-   Return true if *o* is of type :c:data:`PyBool_Type`.
+   Return true if *o* is of type :c:data:`PyBool_Type`.  This function always
+   succeeds.
 
 
 .. c:var:: PyObject* Py_False
diff --git a/Doc/c-api/bytearray.rst b/Doc/c-api/bytearray.rst
index b2f409c15ab..30bcfc7cf9f 100644
--- a/Doc/c-api/bytearray.rst
+++ b/Doc/c-api/bytearray.rst
@@ -25,13 +25,13 @@ Type check macros
 .. c:function:: int PyByteArray_Check(PyObject *o)
 
    Return true if the object *o* is a bytearray object or an instance of a
-   subtype of the bytearray type.
+   subtype of the bytearray type.  This function always succeeds.
 
 
 .. c:function:: int PyByteArray_CheckExact(PyObject *o)
 
    Return true if the object *o* is a bytearray object, but not an instance of a
-   subtype of the bytearray type.
+   subtype of the bytearray type.  This function always succeeds.
 
 
 Direct API functions
diff --git a/Doc/c-api/bytes.rst b/Doc/c-api/bytes.rst
index 0e33ed2c7c9..de65701037a 100644
--- a/Doc/c-api/bytes.rst
+++ b/Doc/c-api/bytes.rst
@@ -25,13 +25,13 @@ called with a non-bytes parameter.
 .. c:function:: int PyBytes_Check(PyObject *o)
 
    Return true if the object *o* is a bytes object or an instance of a subtype
-   of the bytes type.
+   of the bytes type.  This function always succeeds.
 
 
 .. c:function:: int PyBytes_CheckExact(PyObject *o)
 
    Return true if the object *o* is a bytes object, but not an instance of a
-   subtype of the bytes type.
+   subtype of the bytes type.  This function always succeeds.
 
 
 .. c:function:: PyObject* PyBytes_FromString(const char *v)
diff --git a/Doc/c-api/capsule.rst b/Doc/c-api/capsule.rst
index 5eb313c89bf..908e92653dd 100644
--- a/Doc/c-api/capsule.rst
+++ b/Doc/c-api/capsule.rst
@@ -34,7 +34,8 @@ Refer to :ref:`using-capsules` for more information on using these objects.
 
 .. c:function:: int PyCapsule_CheckExact(PyObject *p)
 
-   Return true if its argument is a :c:type:`PyCapsule`.
+   Return true if its argument is a :c:type:`PyCapsule`.  This function always
+   succeeds.
 
 
 .. c:function:: PyObject* PyCapsule_New(void *pointer, const char *name, PyCapsule_Destructor destructor)
diff --git a/Doc/c-api/cell.rst b/Doc/c-api/cell.rst
index 8408f7e398d..ac4ef5adc5c 100644
--- a/Doc/c-api/cell.rst
+++ b/Doc/c-api/cell.rst
@@ -27,7 +27,8 @@ Cell objects are not likely to be useful elsewhere.
 
 .. c:function:: int PyCell_Check(ob)
 
-   Return true if *ob* is a cell object; *ob* must not be ``NULL``.
+   Return true if *ob* is a cell object; *ob* must not be ``NULL``.  This
+   function always succeeds.
 
 
 .. c:function:: PyObject* PyCell_New(PyObject *ob)
diff --git a/Doc/c-api/code.rst b/Doc/c-api/code.rst
index 6f8c41ccbf6..b3a17f1898e 100644
--- a/Doc/c-api/code.rst
+++ b/Doc/c-api/code.rst
@@ -27,7 +27,7 @@ bound into a function.
 
 .. c:function:: int PyCode_Check(PyObject *co)
 
-   Return true if *co* is a :class:`code` object.
+   Return true if *co* is a :class:`code` object.  This function always succeeds.
 
 .. c:function:: int PyCode_GetNumFree(PyCodeObject *co)
 
diff --git a/Doc/c-api/complex.rst b/Doc/c-api/complex.rst
index 06dbb257272..e2ea766b3a3 100644
--- a/Doc/c-api/complex.rst
+++ b/Doc/c-api/complex.rst
@@ -94,13 +94,13 @@ Complex Numbers as Python Objects
 .. c:function:: int PyComplex_Check(PyObject *p)
 
    Return true if its argument is a :c:type:`PyComplexObject` or a subtype of
-   :c:type:`PyComplexObject`.
+   :c:type:`PyComplexObject`.  This function always succeeds.
 
 
 .. c:function:: int PyComplex_CheckExact(PyObject *p)
 
    Return true if its argument is a :c:type:`PyComplexObject`, but not a subtype of
-   :c:type:`PyComplexObject`.
+   :c:type:`PyComplexObject`.  This function always succeeds.
 
 
 .. c:function:: PyObject* PyComplex_FromCComplex(Py_complex v)
diff --git a/Doc/c-api/coro.rst b/Doc/c-api/coro.rst
index 2260944a9a9..caa855a10d2 100644
--- a/Doc/c-api/coro.rst
+++ b/Doc/c-api/coro.rst
@@ -24,6 +24,7 @@ return.
 .. c:function:: int PyCoro_CheckExact(PyObject *ob)
 
    Return true if *ob*'s type is :c:type:`PyCoro_Type`; *ob* must not be ``NULL``.
+   This function always succeeds.
 
 
 .. c:function:: PyObject* PyCoro_New(PyFrameObject *frame, PyObject *name, PyObject *qualname)
diff --git a/Doc/c-api/datetime.rst b/Doc/c-api/datetime.rst
index 415ce4cac67..4c4e4bcfa63 100644
--- a/Doc/c-api/datetime.rst
+++ b/Doc/c-api/datetime.rst
@@ -28,61 +28,66 @@ Type-check macros:
 .. c:function:: int PyDate_Check(PyObject *ob)
 
    Return true if *ob* is of type :c:data:`PyDateTime_DateType` or a subtype of
-   :c:data:`PyDateTime_DateType`.  *ob* must not be ``NULL``.
+   :c:data:`PyDateTime_DateType`.  *ob* must not be ``NULL``.  This function always
+   succeeds.
 
 
 .. c:function:: int PyDate_CheckExact(PyObject *ob)
 
    Return true if *ob* is of type :c:data:`PyDateTime_DateType`. *ob* must not be
-   ``NULL``.
+   ``NULL``.  This function always succeeds.
 
 
 .. c:function:: int PyDateTime_Check(PyObject *ob)
 
    Return true if *ob* is of type :c:data:`PyDateTime_DateTimeType` or a subtype of
-   :c:data:`PyDateTime_DateTimeType`.  *ob* must not be ``NULL``.
+   :c:data:`PyDateTime_DateTimeType`.  *ob* must not be ``NULL``.  This function always
+   succeeds.
 
 
 .. c:function:: int PyDateTime_CheckExact(PyObject *ob)
 
    Return true if *ob* is of type :c:data:`PyDateTime_DateTimeType`. *ob* must not
-   be ``NULL``.
+   be ``NULL``.  This function always succeeds.
 
 
 .. c:function:: int PyTime_Check(PyObject *ob)
 
    Return true if *ob* is of type :c:data:`PyDateTime_TimeType` or a subtype of
-   :c:data:`PyDateTime_TimeType`.  *ob* must not be ``NULL``.
+   :c:data:`PyDateTime_TimeType`.  *ob* must not be ``NULL``.  This function always
+   succeeds.
 
 
 .. c:function:: int PyTime_CheckExact(PyObject *ob)
 
    Return true if *ob* is of type :c:data:`PyDateTime_TimeType`. *ob* must not be
-   ``NULL``.
+   ``NULL``.  This function always succeeds.
 
 
 .. c:function:: int PyDelta_Check(PyObject *ob)
 
    Return true if *ob* is of type :c:data:`PyDateTime_DeltaType` or a subtype of
-   :c:data:`PyDateTime_DeltaType`.  *ob* must not be ``NULL``.
+   :c:data:`PyDateTime_DeltaType`.  *ob* must not be ``NULL``.  This function always
+   succeeds.
 
 
 .. c:function:: int PyDelta_CheckExact(PyObject *ob)
 
    Return true if *ob* is of type :c:data:`PyDateTime_DeltaType`. *ob* must not be
-   ``NULL``.
+   ``NULL``.  This function always succeeds.
 
 
 .. c:function:: int PyTZInfo_Check(PyObject *ob)
 
    Return true if *ob* is of type :c:data:`PyDateTime_TZInfoType` or a subtype of
-   :c:data:`PyDateTime_TZInfoType`.  *ob* must not be ``NULL``.
+   :c:data:`PyDateTime_TZInfoType`.  *ob* must not be ``NULL``.  This function always
+   succeeds.
 
 
 .. c:function:: int PyTZInfo_CheckExact(PyObject *ob)
 
    Return true if *ob* is of type :c:data:`PyDateTime_TZInfoType`. *ob* must not be
-   ``NULL``.
+   ``NULL``.  This function always succeeds.
 
 
 Macros to create objects:
diff --git a/Doc/c-api/dict.rst b/Doc/c-api/dict.rst
index 8e0d6845463..d257c9b5f76 100644
--- a/Doc/c-api/dict.rst
+++ b/Doc/c-api/dict.rst
@@ -22,13 +22,13 @@ Dictionary Objects
 .. c:function:: int PyDict_Check(PyObject *p)
 
    Return true if *p* is a dict object or an instance of a subtype of the dict
-   type.
+   type.  This function always succeeds.
 
 
 .. c:function:: int PyDict_CheckExact(PyObject *p)
 
    Return true if *p* is a dict object, but not an instance of a subtype of
-   the dict type.
+   the dict type.  This function always succeeds.
 
 
 .. c:function:: PyObject* PyDict_New()
diff --git a/Doc/c-api/float.rst b/Doc/c-api/float.rst
index b29937dbecd..c107243a88d 100644
--- a/Doc/c-api/float.rst
+++ b/Doc/c-api/float.rst
@@ -22,13 +22,13 @@ Floating Point Objects
 .. c:function:: int PyFloat_Check(PyObject *p)
 
    Return true if its argument is a :c:type:`PyFloatObject` or a subtype of
-   :c:type:`PyFloatObject`.
+   :c:type:`PyFloatObject`.  This function always succeeds.
 
 
 .. c:function:: int PyFloat_CheckExact(PyObject *p)
 
    Return true if its argument is a :c:type:`PyFloatObject`, but not a subtype of
-   :c:type:`PyFloatObject`.
+   :c:type:`PyFloatObject`.  This function always succeeds.
 
 
 .. c:function:: PyObject* PyFloat_FromString(PyObject *str)
diff --git a/Doc/c-api/function.rst b/Doc/c-api/function.rst
index bb416f4bb63..20968828e0b 100644
--- a/Doc/c-api/function.rst
+++ b/Doc/c-api/function.rst
@@ -26,7 +26,7 @@ There are a few functions specific to Python functions.
 .. c:function:: int PyFunction_Check(PyObject *o)
 
    Return true if *o* is a function object (has type :c:data:`PyFunction_Type`).
-   The parameter must not be ``NULL``.
+   The parameter must not be ``NULL``.  This function always succeeds.
 
 
 .. c:function:: PyObject* PyFunction_New(PyObject *code, PyObject *globals)
diff --git a/Doc/c-api/gen.rst b/Doc/c-api/gen.rst
index 74410927bfd..0eb5922f6da 100644
--- a/Doc/c-api/gen.rst
+++ b/Doc/c-api/gen.rst
@@ -22,12 +22,14 @@ than explicitly calling :c:func:`PyGen_New` or :c:func:`PyGen_NewWithQualName`.
 
 .. c:function:: int PyGen_Check(PyObject *ob)
 
-   Return true if *ob* is a generator object; *ob* must not be ``NULL``.
+   Return true if *ob* is a generator object; *ob* must not be ``NULL``.  This
+   function always succeeds.
 
 
 .. c:function:: int PyGen_CheckExact(PyObject *ob)
 
-   Return true if *ob*'s type is :c:type:`PyGen_Type`; *ob* must not be ``NULL``.
+   Return true if *ob*'s type is :c:type:`PyGen_Type`; *ob* must not be
+   ``NULL``.  This function always succeeds.
 
 
 .. c:function:: PyObject* PyGen_New(PyFrameObject *frame)
diff --git a/Doc/c-api/iter.rst b/Doc/c-api/iter.rst
index 68df6f6e89f..74fb5578abd 100644
--- a/Doc/c-api/iter.rst
+++ b/Doc/c-api/iter.rst
@@ -9,7 +9,8 @@ There are two functions specifically for working with iterators.
 
 .. c:function:: int PyIter_Check(PyObject *o)
 
-   Return true if the object *o* supports the iterator protocol.
+   Return true if the object *o* supports the iterator protocol.  This
+   function always succeeds.
 
 
 .. c:function:: PyObject* PyIter_Next(PyObject *o)
diff --git a/Doc/c-api/iterator.rst b/Doc/c-api/iterator.rst
index 4d91e4a224f..3fcf099134d 100644
--- a/Doc/c-api/iterator.rst
+++ b/Doc/c-api/iterator.rst
@@ -21,7 +21,8 @@ sentinel value is returned.
 
 .. c:function:: int PySeqIter_Check(op)
 
-   Return true if the type of *op* is :c:data:`PySeqIter_Type`.
+   Return true if the type of *op* is :c:data:`PySeqIter_Type`.  This function
+   always succeeds.
 
 
 .. c:function:: PyObject* PySeqIter_New(PyObject *seq)
@@ -39,7 +40,8 @@ sentinel value is returned.
 
 .. c:function:: int PyCallIter_Check(op)
 
-   Return true if the type of *op* is :c:data:`PyCallIter_Type`.
+   Return true if the type of *op* is :c:data:`PyCallIter_Type`.  This
+   function always succeeds.
 
 
 .. c:function:: PyObject* PyCallIter_New(PyObject *callable, PyObject *sentinel)
diff --git a/Doc/c-api/list.rst b/Doc/c-api/list.rst
index 0bc0785f200..f338e2ae066 100644
--- a/Doc/c-api/list.rst
+++ b/Doc/c-api/list.rst
@@ -22,13 +22,13 @@ List Objects
 .. c:function:: int PyList_Check(PyObject *p)
 
    Return true if *p* is a list object or an instance of a subtype of the list
-   type.
+   type.  This function always succeeds.
 
 
 .. c:function:: int PyList_CheckExact(PyObject *p)
 
    Return true if *p* is a list object, but not an instance of a subtype of
-   the list type.
+   the list type.  This function always succeeds.
 
 
 .. c:function:: PyObject* PyList_New(Py_ssize_t len)
diff --git a/Doc/c-api/long.rst b/Doc/c-api/long.rst
index 4d859fb641d..4201490286b 100644
--- a/Doc/c-api/long.rst
+++ b/Doc/c-api/long.rst
@@ -27,13 +27,13 @@ distinguished from a number.  Use :c:func:`PyErr_Occurred` to disambiguate.
 .. c:function:: int PyLong_Check(PyObject *p)
 
    Return true if its argument is a :c:type:`PyLongObject` or a subtype of
-   :c:type:`PyLongObject`.
+   :c:type:`PyLongObject`.  This function always succeeds.
 
 
 .. c:function:: int PyLong_CheckExact(PyObject *p)
 
    Return true if its argument is a :c:type:`PyLongObject`, but not a subtype of
-   :c:type:`PyLongObject`.
+   :c:type:`PyLongObject`.  This function always succeeds.
 
 
 .. c:function:: PyObject* PyLong_FromLong(long v)
diff --git a/Doc/c-api/memoryview.rst b/Doc/c-api/memoryview.rst
index de429e5c11d..24f8c935302 100644
--- a/Doc/c-api/memoryview.rst
+++ b/Doc/c-api/memoryview.rst
@@ -45,7 +45,8 @@ any other object.
 .. c:function:: int PyMemoryView_Check(PyObject *obj)
 
    Return true if the object *obj* is a memoryview object.  It is not
-   currently allowed to create subclasses of :class:`memoryview`.
+   currently allowed to create subclasses of :class:`memoryview`.  This
+   function always succeeds.
 
 
 .. c:function:: Py_buffer *PyMemoryView_GET_BUFFER(PyObject *mview)
diff --git a/Doc/c-api/method.rst b/Doc/c-api/method.rst
index 0a5341cbbdf..23852251dfe 100644
--- a/Doc/c-api/method.rst
+++ b/Doc/c-api/method.rst
@@ -22,6 +22,7 @@ to bind a :c:data:`PyCFunction` to a class object. It replaces the former call
 
    Return true if *o* is an instance method object (has type
    :c:data:`PyInstanceMethod_Type`).  The parameter must not be ``NULL``.
+   This function always succeeds.
 
 
 .. c:function:: PyObject* PyInstanceMethod_New(PyObject *func)
@@ -64,7 +65,7 @@ no longer available.
 .. c:function:: int PyMethod_Check(PyObject *o)
 
    Return true if *o* is a method object (has type :c:data:`PyMethod_Type`).  The
-   parameter must not be ``NULL``.
+   parameter must not be ``NULL``.  This function always succeeds.
 
 
 .. c:function:: PyObject* PyMethod_New(PyObject *func, PyObject *self)
diff --git a/Doc/c-api/module.rst b/Doc/c-api/module.rst
index 41a705d9e99..a2541afb685 100644
--- a/Doc/c-api/module.rst
+++ b/Doc/c-api/module.rst
@@ -19,12 +19,13 @@ Module Objects
 .. c:function:: int PyModule_Check(PyObject *p)
 
    Return true if *p* is a module object, or a subtype of a module object.
+   This function always succeeds.
 
 
 .. c:function:: int PyModule_CheckExact(PyObject *p)
 
    Return true if *p* is a module object, but not a subtype of
-   :c:data:`PyModule_Type`.
+   :c:data:`PyModule_Type`.  This function always succeeds.
 
 
 .. c:function:: PyObject* PyModule_NewObject(PyObject *name)
diff --git a/Doc/c-api/set.rst b/Doc/c-api/set.rst
index 879f394d966..84f34e7dae8 100644
--- a/Doc/c-api/set.rst
+++ b/Doc/c-api/set.rst
@@ -53,28 +53,29 @@ the constructor functions work with any iterable Python object.
 .. c:function:: int PySet_Check(PyObject *p)
 
    Return true if *p* is a :class:`set` object or an instance of a subtype.
+   This function always succeeds.
 
 .. c:function:: int PyFrozenSet_Check(PyObject *p)
 
    Return true if *p* is a :class:`frozenset` object or an instance of a
-   subtype.
+   subtype.  This function always succeeds.
 
 .. c:function:: int PyAnySet_Check(PyObject *p)
 
    Return true if *p* is a :class:`set` object, a :class:`frozenset` object, or an
-   instance of a subtype.
+   instance of a subtype.  This function always succeeds.
 
 
 .. c:function:: int PyAnySet_CheckExact(PyObject *p)
 
    Return true if *p* is a :class:`set` object or a :class:`frozenset` object but
-   not an instance of a subtype.
+   not an instance of a subtype.  This function always succeeds.
 
 
 .. c:function:: int PyFrozenSet_CheckExact(PyObject *p)
 
    Return true if *p* is a :class:`frozenset` object but not an instance of a
-   subtype.
+   subtype.  This function always succeeds.
 
 
 .. c:function:: PyObject* PySet_New(PyObject *iterable)
diff --git a/Doc/c-api/slice.rst b/Doc/c-api/slice.rst
index 48a58c6c6f7..8271d9acfb6 100644
--- a/Doc/c-api/slice.rst
+++ b/Doc/c-api/slice.rst
@@ -14,7 +14,8 @@ Slice Objects
 
 .. c:function:: int PySlice_Check(PyObject *ob)
 
-   Return true if *ob* is a slice object; *ob* must not be ``NULL``.
+   Return true if *ob* is a slice object; *ob* must not be ``NULL``.  This
+   function always succeeds.
 
 
 .. c:function:: PyObject* PySlice_New(PyObject *start, PyObject *stop, PyObject *step)
diff --git a/Doc/c-api/tuple.rst b/Doc/c-api/tuple.rst
index c0c14f480d3..6919e610227 100644
--- a/Doc/c-api/tuple.rst
+++ b/Doc/c-api/tuple.rst
@@ -21,14 +21,14 @@ Tuple Objects
 
 .. c:function:: int PyTuple_Check(PyObject *p)
 
-   Return true if *p* is a tuple object or an instance of a subtype of the tuple
-   type.
+   Return true if *p* is a tuple object or an instance of a subtype of the
+   tuple type.  This function always succeeds.
 
 
 .. c:function:: int PyTuple_CheckExact(PyObject *p)
 
    Return true if *p* is a tuple object, but not an instance of a subtype of the
-   tuple type.
+   tuple type.  This function always succeeds.
 
 
 .. c:function:: PyObject* PyTuple_New(Py_ssize_t len)
diff --git a/Doc/c-api/type.rst b/Doc/c-api/type.rst
index a869859dd42..b9bfa554e81 100644
--- a/Doc/c-api/type.rst
+++ b/Doc/c-api/type.rst
@@ -23,12 +23,14 @@ Type Objects
 
    Return non-zero if the object *o* is a type object, including instances of
    types derived from the standard type object.  Return 0 in all other cases.
+   This function always succeeds.
 
 
 .. c:function:: int PyType_CheckExact(PyObject *o)
 
-   Return non-zero if the object *o* is a type object, but not a subtype of the
-   standard type object.  Return 0 in all other cases.
+   Return non-zero if the object *o* is a type object, but not a subtype of
+   the standard type object.  Return 0 in all other cases.  This function
+   always succeeds.
 
 
 .. c:function:: unsigned int PyType_ClearCache()
diff --git a/Doc/c-api/unicode.rst b/Doc/c-api/unicode.rst
index b7f99d32558..1851c645f41 100644
--- a/Doc/c-api/unicode.rst
+++ b/Doc/c-api/unicode.rst
@@ -91,13 +91,13 @@ access internal read-only data of Unicode objects:
 .. c:function:: int PyUnicode_Check(PyObject *o)
 
    Return true if the object *o* is a Unicode object or an instance of a Unicode
-   subtype.
+   subtype.  This function always succeeds.
 
 
 .. c:function:: int PyUnicode_CheckExact(PyObject *o)
 
    Return true if the object *o* is a Unicode object, but not an instance of a
-   subtype.
+   subtype.  This function always succeeds.
 
 
 .. c:function:: int PyUnicode_READY(PyObject *o)
diff --git a/Doc/c-api/weakref.rst b/Doc/c-api/weakref.rst
index 9205ca620a6..98ebe711ada 100644
--- a/Doc/c-api/weakref.rst
+++ b/Doc/c-api/weakref.rst
@@ -13,17 +13,18 @@ as much as it can.
 
 .. c:function:: int PyWeakref_Check(ob)
 
-   Return true if *ob* is either a reference or proxy object.
+   Return true if *ob* is either a reference or proxy object.  This function
+   always succeeds.
 
 
 .. c:function:: int PyWeakref_CheckRef(ob)
 
-   Return true if *ob* is a reference object.
+   Return true if *ob* is a reference object.  This function always succeeds.
 
 
 .. c:function:: int PyWeakref_CheckProxy(ob)
 
-   Return true if *ob* is a proxy object.
+   Return true if *ob* is a proxy object.  This function always succeeds.
 
 
 .. c:function:: PyObject* PyWeakref_NewRef(PyObject *ob, PyObject *callback)