[3.13] gh-136437: Document os.path.dirname as accepting only pos-only (GH-136946) (#136948)

gh-136437: Document `os.path.dirname` as accepting only pos-only (GH-136946)
(cherry picked from commit 3224429)

Co-authored-by: sobolevn <[email protected]>

Author: miss-islington

Doc/library/os.path.rst

@@ -118,7 +118,7 @@ the :mod:`glob` module.)
       Accepts a :term:`path-like object`.
 
 
-.. function:: dirname(path)
+.. function:: dirname(path, /)
 
    Return the directory name of pathname *path*.  This is the first element of
    the pair returned by passing *path* to the function :func:`split`.