Improve FRAME storage docs (#1714)
This is a port (and hopefully a small improvement) of @Kianenigma 's PR from the old Substrate repo: https://github.com/paritytech/substrate/pull/13987. Following #1689 I moved the documentation of all macros relevant to this PR from `frame_support_procedural` to `pallet_macros` while including a hint for RA users. Question: Again with respect to #1689: Is there a good reason why we should *not* enhance paths with links to our current rustdocs? For example, instead of ```rust /// **Rust-Analyzer users**: See the documentation of the Rust item in /// `frame_support::pallet_macros::storage`. ``` we could write ```rust /// **Rust-Analyzer users**: See the documentation of the Rust item in /// [`frame_support::pallet_macros::storage`](https://paritytech.github.io/polkadot-sdk/master/frame_support/pallet_macros/attr.storage.html). ``` This results in a clickable link like this: <img width="674" alt="image" src="https://github.com/paritytech/polkadot-sdk/assets/10713977/c129e622-3942-4eeb-8acf-93ee4efdc99d"> I don't really expect the links to become outdated any time soon, but I think this would be a great UX improvement over just having paths. TODOs: - [ ] Add documentation for `constant_name` macro - [x] Add proper documentation for different `QueryKinds`, i.e. `OptionQuery`, `ValueQuery`, `ResultQuery`. One example for each. Custom `OnEmpty` should be moved to `QueryKinds` trait doc page. - [ ] Rework `type_value` docs --------- Co-authored-by:kianenigma <[email protected]>