Where rfc822.Message is mentioned, add a link to the rfc822 module.

Doc/lib/libmailbox.tex

@@ -44,9 +44,10 @@ All implementations of Mailbox objects have one externally visible
 method:
 
 \begin{methoddesc}[mailbox]{next}{}
-Return the next message in the mailbox, as a \class{rfc822.Message} object.
-Depending on the mailbox implementation the \var{fp} attribute of this
-object may be a true file object or a class instance simulating a file object,
-taking care of things like message boundaries if multiple mail messages are
+Return the next message in the mailbox, as a \class{rfc822.Message}
+object (see the \refmodule{rfc822} module).  Depending on the mailbox
+implementation the \var{fp} attribute of this object may be a true
+file object or a class instance simulating a file object, taking care
+of things like message boundaries if multiple mail messages are
 contained in a single file, etc.
 \end{methoddesc}