updated librsync docs

[Martin Pool]

Hi all,

I wanted to let you know that I've done a big update to the librsync docs, which are available at http://librsync.sourcefrog.net/ and in the source tree.

This included:

 - merged the ReST and Doxygen docs

 - added a bunch of documentation including of the callback API

 - better use of Doxygen structure features

 - deleted some incorrect or unhelpful docs

--

Martin