Merge 3.4 (os doc)

Doc/library/os.rst

@@ -2604,8 +2604,9 @@ features:
           if 'CVS' in dirs:
               dirs.remove('CVS')  # don't visit CVS directories
 
-   In the next example, walking the tree bottom-up is essential: :func:`rmdir`
-   doesn't allow deleting a directory before the directory is empty::
+   In the next example (simple implementation of :func:`shutil.rmtree`),
+   walking the tree bottom-up is essential, :func:`rmdir` doesn't allow
+   deleting a directory before the directory is empty::
 
       # Delete everything reachable from the directory named in "top",
       # assuming there are no symbolic links.