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).
```
**Specific parameters**, used to validate the content. Only one of them should be specified.
-       * *fields*: A list of fields like in a model definition.
-       * *model*: The id of an existing model.
```