(International): Add an overview of Mule features, with pointers to

(International): Add an overview of Mule features, with pointers to
detailed description.
(Enabling Multibyte): Describe how to switch a unibyte session to multibyte.
Mention that by default, all sessions are multibyte.
(Coding Systems): Make it clear that cpNNN are coding systems, and should
be used as such.
(Recognize Coding): Explain that Emacs decodes text as part of reading
it.  Mention revert-buffer as a means to redecode a file.