Improve the documentation of Syndic.Rss2
Improve the documentation of Syndic.Atom

In particular, the constraints enforced by the type system have not
been restated in the documentation.
Remove superfluous export of types

Fixes https://github.com/Cumulus/Syndic/issues/9
Merge pull request #8 from Chris00/master

Improve the documentation (cont'd)
Merge master into gh-pages
Update documentation 36