Derive macro docs should contain or link to attribute docs

[VorpalBlade]

https://docs.rs/serde/latest/serde/derive.Serialize.html (and the page for Derserialize) is fairly useless. It would be nice to document what the syntax is.

I do understand not wanting duplicate docs but linking to the relevant docs does not seem to be an overly large maintenance burden. I would suggest something like:

For an introduction see https://serde.rs/derive.html. For accepted attributes on the container and member fields see https://serde.rs/attributes.html.

(I would prefer to have the docs fully in rustdoc, since that can be easily generated locally when offline and the crate has been downloaded earlier, but since that is not the case, let's make the best of the current situation.)