Merge #15342: Add clarifying sentence to posixpath.join docstring.

This sentence appears as a clarifying note in the HTML docs, and seems
worth having in the docstring since it covers a very common use case
that isn't otherwise obvious.  Thanks to Yongzhi Pan for the suggestion.

Lib/posixpath.py

@@ -71,7 +71,8 @@ def isabs(s):
 def join(a, *p):
     """Join two or more pathname components, inserting '/' as needed.
     If any component is an absolute path, all previous path components
-    will be discarded."""
+    will be discarded.  An empty last part will result in a path that
+    ends with a separator."""
     sep = _get_sep(a)
     path = a
     try: