bpo-26329: update os.path.normpath documentation (pythonGH-20138)

* bpo-26329: update os.path.normpath documentation

* Update os.path.rst

* Update posixpath.py

* update Pathname Resolution note

Doc/library/os.path.rst

@@ -340,6 +340,14 @@ the :mod:`glob` module.)
    that contains symbolic links.  On Windows, it converts forward slashes to
    backward slashes. To normalize case, use :func:`normcase`.
 
+  .. note::
+      On POSIX systems, in accordance with `IEEE Std 1003.1 2013 Edition; 4.13
+      Pathname Resolution <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_13>`_,
+      if a pathname begins with exactly two slashes, the first component
+      following the leading characters may be interpreted in an implementation-defined
+      manner, although more than two leading characters shall be treated as a
+      single character.
+
    .. versionchanged:: 3.6
       Accepts a :term:`path-like object`.
 

Lib/posixpath.py

@@ -352,6 +352,7 @@ def normpath(path):
     initial_slashes = path.startswith(sep)
     # POSIX allows one or two initial slashes, but treats three or more
     # as single slash.
+    # (see http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_13)
     if (initial_slashes and
         path.startswith(sep*2) and not path.startswith(sep*3)):
         initial_slashes = 2