gh-160904: Correct os.path.normcase documentation for POSIX

os.path.normcase does not do nothing on POSIX: it calls os.fspath,
which normalizes path-like objects to str or bytes. Say so in the
docstring and in the documentation, instead of claiming that the
path is returned unchanged.

Author: GUOZIJAN

Doc/library/os.path.rst

@@ -415,7 +415,9 @@ the :mod:`glob` module.)
 
    Normalize the case of a pathname.  On Windows, convert all characters in the
    pathname to lowercase, and also convert forward slashes to backward slashes.
-   On other operating systems, return the path unchanged.
+   On other operating systems, do not change the case of the path, but
+   normalize :term:`path-like objects <path-like object>` to :class:`str` or
+   :class:`bytes`, just like :func:`os.fspath`.
 
    .. versionchanged:: 3.6
       Accepts a :term:`path-like object`.

Lib/posixpath.py

@@ -52,7 +52,8 @@ def _get_sep(path):
 # (another function should be defined to do that).
 
 def normcase(s, /):
-    """Normalize case of pathname.  Has no effect under Posix"""
+    """Normalize case of pathname.  Does not change the case, but
+    normalizes path-like objects to str or bytes, like os.fspath"""
     return os.fspath(s)