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).
Ok, following https://github.com/spiral-project/daybed/pull/254#discussion_r19715473, and after reading the tests, it seems it's either the model OR the fields, not both, and not none ;)