Document macros

[axelkar]

The attribute and derive macros are not currently documented in the Rustdoc: https://docs.rs/salsa/latest/salsa/#attributes

It'd be useful to have a reference for the macros. For example, there's no documentation stating what #[salsa::db] does to traits, except a short line in salsa::Database describing that it can create subtraits.

[MichaReiser]

Yes, having some documentation on the macros and what attributes they support would be great. I'd happily accept a PR for this.