SF bug [#469732] os.path.walk docstring inconsistent.

We have 5 implementations of walk(), and 5 different docstrings.  Combined
'em.  Let's see how long it takes before they're all different again!

Lib/macpath.py

@@ -201,13 +201,19 @@ def normpath(s):
 
 
 def walk(top, func, arg):
-    """Directory tree walk.
-    For each directory under top (including top itself),
-    func(arg, dirname, filenames) is called, where
-    dirname is the name of the directory and filenames is the list
-    of files (and subdirectories etc.) in the directory.
-    The func may modify the filenames list, to implement a filter,
-    or to impose a different order of visiting."""
+    """Directory tree walk with callback function.
+
+    For each directory in the directory tree rooted at top (including top
+    itself, but excluding '.' and '..'), call func(arg, dirname, fnames).
+    dirname is the name of the directory, and fnames a list of the names of
+    the files and subdirectories in dirname (excluding '.' and '..').  func
+    may modify the fnames list in-place (e.g. via del or slice assignment),
+    and walk will only recurse into the subdirectories whose names remain in
+    fnames; this can be used to implement a filter, or to impose a specific
+    order of visiting.  No semantics are defined for, or required of, arg,
+    beyond that arg is always passed to func.  It can be used, e.g., to pass
+    a filename pattern, or a mutable object designed to accumulate
+    statistics.  Passing None for arg is common."""
 
     try:
         names = os.listdir(top)