Add note advising that 'filter' be used as a keyword-argument.

Doc/library/tarfile.rst

@@ -419,7 +419,9 @@ be finalized; only the internally used file object will be closed. See the
 
    .. deprecated:: 2.7
       The *exclude* parameter is deprecated, please use the *filter* parameter
-      instead.
+      instead.  For maximum portability, *filter* should be used as a keyword
+      argument rather than as a positional argument so that code won't be
+      affected when *exclude* is ultimately removed.
 
 
 .. method:: TarFile.addfile(tarinfo, fileobj=None)