Issue 17447: Clarify that str.isidentifier doesn't check for reserved keywords.

Doc/library/stdtypes.rst

@@ -1627,6 +1627,8 @@ expression support in the :mod:`re` module).
    Return true if the string is a valid identifier according to the language
    definition, section :ref:`identifiers`.
 
+   Use :func:`keyword.iskeyword` to test for reserved identifiers such as
+   :keyword:`def` and :keyword:`class`.
 
 .. method:: str.islower()
 

Objects/unicodeobject.c

@@ -11493,7 +11493,10 @@ PyDoc_STRVAR(isidentifier__doc__,
              "S.isidentifier() -> bool\n\
 \n\
 Return True if S is a valid identifier according\n\
-to the language definition.");
+to the language definition.\n\
+\n\
+Use keyword.iskeyword() to test for reserved identifiers\n\
+such as \"def\" and \"class\".\n");
 
 static PyObject*
 unicode_isidentifier(PyObject *self)