Err I forgot to add details..

Hi Guix,

This is my first patch to Guix. Janneke asked me to fill in the documentation for `svn-fetch`.

This patch should contain the documentation for:

 - `hg-reference`
 - `svn-fetch`
 - `svn-reference`
 - `bzr-fetch`
 - `bzr-reference`

This should now cover all currently supported origin methods:

 - local
 - Git
 - Mercurial
 - Subversion
 - Bazaar

One thing I was unsure about was whether `svn-reference` had a field `recursive?`. However, from what I could find, Subversion has no equivalent to Git's sub-modules. However, I documented Subversion's `recursive?` with what it would do if it did anything sensible.

I would be delighted to make any corrections needed!

Kind regards,
 - Christina O'Donnell