Added reference to the multifile module from the "See also" section

in the mimetools documentation.
Suggested by Skip Montanaro <skip@mojam.com>.

Doc/lib/libmimetools.tex

@@ -51,6 +51,8 @@ open file \var{output}.  The block size is currently fixed at 8192.
 \begin{seealso}
   \seemodule{rfc822}{Provides the base class for
                      \class{mimetools.Message}.}
+  \seemodule{multifile}{Support for reading files which contain
+                        distinct parts, such as MIME data.}
 \end{seealso}