Some documentation improvements

Please proof-read this very carefully, documentation is important, and I don't
know much about daybed yet, so I could very possibly be wrong about some of
those modifications.

Also, if you have a better idea than replacing all the `v1` everywhere in the
URLs on the next version (like using a variable or something), please let me
know (i've checked ext_link and rst_epilog, but none seem to do the job... or
be usable in code blocks).
This paragraph looks superfluous...

Why not just:

>  Like the ``group`` field type, it can help to build the form layout.