Thanks.  Looks good except that it should check isinstance(string) rather than isinstance(Charset), that way someone can pass a custom class that implements the Charset API if they want.  (Alternatively, the check could be for an encode_header method...actually that might be better, although it isn't what the other email modules do.)

The doc strings are an abreviated version of what is in the docstrings, and the text is often not-quite-equivalent even when it is not a strict subset of the docs.  We believe it produces higher quality documentation to maintain them separately and tune each one for its intended use case (though this does mean that they occasionally get out of sync due to oversights).