bpo-40204: Fix duplicates in the documentation (GH-21857)

Fix two Sphinx 3 issues:

Doc/c-api/buffer.rst:304: WARNING: Duplicate C declaration, also defined in 'c-api/buffer'.
Declaration is 'PyBUF_ND'.

Doc/c-api/unicode.rst:1603: WARNING: Duplicate C declaration, also defined in 'c-api/unicode'.
Declaration is 'PyObject* PyUnicode_Translate(PyObject *str, PyObject *table, const char *errors)'.

Doc/c-api/buffer.rst

@@ -301,7 +301,7 @@ must be C-contiguous.
 +-----------------------------------+-------+---------+------------+--------+
 | .. c:macro:: PyBUF_ANY_CONTIGUOUS |  yes  |   yes   |    NULL    | C or F |
 +-----------------------------------+-------+---------+------------+--------+
-| .. c:macro:: PyBUF_ND             |  yes  |   NULL  |    NULL    |   C    |
+| :c:macro:`PyBUF_ND`               |  yes  |   NULL  |    NULL    |   C    |
 +-----------------------------------+-------+---------+------------+--------+
 
 

Doc/c-api/unicode.rst

@@ -1487,17 +1487,21 @@ These are the mapping codec APIs:
 
 The following codec API is special in that maps Unicode to Unicode.
 
-.. c:function:: PyObject* PyUnicode_Translate(PyObject *unicode, \
+.. c:function:: PyObject* PyUnicode_Translate(PyObject *str, PyObject *table, const char *errors)
-                              PyObject *mapping, const char *errors)
 
-   Translate a Unicode object using the given *mapping* object and return the
+   Translate a string by applying a character mapping table to it and return the
-   resulting Unicode object.  Return ``NULL`` if an exception was raised by the
+   resulting Unicode object. Return ``NULL`` if an exception was raised by the
    codec.
 
-   The *mapping* object must map Unicode ordinal integers to Unicode strings,
+   The mapping table must map Unicode ordinal integers to Unicode ordinal integers
-   integers (which are then interpreted as Unicode ordinals) or ``None``
+   or ``None`` (causing deletion of the character).
-   (causing deletion of the character).  Unmapped character ordinals (ones
+
-   which cause a :exc:`LookupError`) are left untouched and are copied as-is.
+   Mapping tables need only provide the :meth:`__getitem__` interface; dictionaries
+   and sequences work well.  Unmapped character ordinals (ones which cause a
+   :exc:`LookupError`) are left untouched and are copied as-is.
+
+   *errors* has the usual meaning for codecs. It may be ``NULL`` which indicates to
+   use the default error handling.
 
 
 .. c:function:: PyObject* PyUnicode_TranslateCharmap(const Py_UNICODE *s, Py_ssize_t size, \
@@ -1600,23 +1604,6 @@ They all return ``NULL`` or ``-1`` if an exception occurs.
    characters are not included in the resulting strings.
 
 
-.. c:function:: PyObject* PyUnicode_Translate(PyObject *str, PyObject *table, \
-                              const char *errors)
-
-   Translate a string by applying a character mapping table to it and return the
-   resulting Unicode object.
-
-   The mapping table must map Unicode ordinal integers to Unicode ordinal integers
-   or ``None`` (causing deletion of the character).
-
-   Mapping tables need only provide the :meth:`__getitem__` interface; dictionaries
-   and sequences work well.  Unmapped character ordinals (ones which cause a
-   :exc:`LookupError`) are left untouched and are copied as-is.
-
-   *errors* has the usual meaning for codecs. It may be ``NULL`` which indicates to
-   use the default error handling.
-
-
 .. c:function:: PyObject* PyUnicode_Join(PyObject *separator, PyObject *seq)
 
    Join a sequence of strings using the given *separator* and return the resulting