Restore PyObject_IsInstance() comment (GH-18345)

Restore PyObject_IsInstance() comment explaining why only tuples of
types are accepted, but not general sequence. Comment written by
Guido van Rossum in commit 03290ecbf1
which implements isinstance(x, (A, B, ...)). The comment was lost in
a PyObject_IsInstance() optimization:
commit ec569b7947.

Cleanup also the code. recursive_isinstance() is no longer recursive,
so rename it to object_isinstance(), whereas object_isinstance() is
recursive and so rename it to object_recursive_isinstance().

Objects/abstract.c

@@ -2423,7 +2423,7 @@ check_class(PyObject *cls, const char *error)
 }
 
 static int
-recursive_isinstance(PyObject *inst, PyObject *cls)
+object_isinstance(PyObject *inst, PyObject *cls)
 {
     PyObject *icls;
     int retval;
@@ -2461,21 +2461,23 @@ recursive_isinstance(PyObject *inst, PyObject *cls)
 }
 
 static int
-object_isinstance(PyThreadState *tstate, PyObject *inst, PyObject *cls)
+object_recursive_isinstance(PyThreadState *tstate, PyObject *inst, PyObject *cls)
 {
     _Py_IDENTIFIER(__instancecheck__);
-    PyObject *checker;
 
     /* Quick test for an exact match */
-    if (Py_TYPE(inst) == (PyTypeObject *)cls)
+    if (Py_TYPE(inst) == (PyTypeObject *)cls) {
         return 1;
+    }
 
     /* We know what type's __instancecheck__ does. */
     if (PyType_CheckExact(cls)) {
-        return recursive_isinstance(inst, cls);
+        return object_isinstance(inst, cls);
     }
 
     if (PyTuple_Check(cls)) {
+        /* Not a general sequence -- that opens up the road to
+           recursion and stack overflow. */
         if (_Py_EnterRecursiveCall(tstate, " in __instancecheck__")) {
             return -1;
         }
@@ -2483,37 +2485,41 @@ object_isinstance(PyThreadState *tstate, PyObject *inst, PyObject *cls)
         int r = 0;
         for (Py_ssize_t i = 0; i < n; ++i) {
             PyObject *item = PyTuple_GET_ITEM(cls, i);
-            r = object_isinstance(tstate, inst, item);
-            if (r != 0)
+            r = object_recursive_isinstance(tstate, inst, item);
+            if (r != 0) {
                 /* either found it, or got an error */
                 break;
             }
+        }
         _Py_LeaveRecursiveCall(tstate);
         return r;
     }
 
-    checker = _PyObject_LookupSpecial(cls, &PyId___instancecheck__);
+    PyObject *checker = _PyObject_LookupSpecial(cls, &PyId___instancecheck__);
     if (checker != NULL) {
-        int ok = -1;
         if (_Py_EnterRecursiveCall(tstate, " in __instancecheck__")) {
             Py_DECREF(checker);
-            return ok;
+            return -1;
         }
+
         PyObject *res = _PyObject_CallOneArg(checker, inst);
         _Py_LeaveRecursiveCall(tstate);
         Py_DECREF(checker);
-        if (res != NULL) {
-            ok = PyObject_IsTrue(res);
-            Py_DECREF(res);
+
+        if (res == NULL) {
+            return -1;
         }
+        int ok = PyObject_IsTrue(res);
+        Py_DECREF(res);
+
         return ok;
     }
     else if (_PyErr_Occurred(tstate)) {
         return -1;
     }
 
-    /* Probably never reached anymore. */
-    return recursive_isinstance(inst, cls);
+    /* cls has no __instancecheck__() method */
+    return object_isinstance(inst, cls);
 }
 
 
@@ -2521,7 +2527,7 @@ int
 PyObject_IsInstance(PyObject *inst, PyObject *cls)
 {
     PyThreadState *tstate = _PyThreadState_GET();
-    return object_isinstance(tstate, inst, cls);
+    return object_recursive_isinstance(tstate, inst, cls);
 }
 
 
@@ -2611,7 +2617,7 @@ PyObject_IsSubclass(PyObject *derived, PyObject *cls)
 int
 _PyObject_RealIsInstance(PyObject *inst, PyObject *cls)
 {
-    return recursive_isinstance(inst, cls);
+    return object_isinstance(inst, cls);
 }
 
 int