update genesis_config related docs and tests and error messages (#1642)

Follow-up to https://github.com/paritytech/substrate/pull/14306.

I hope this also showcases the important message of: **It is really not
that hard to make the examples codes in rust-docs compile, and therefore
remain correct. Please embrace this :)**

It moves the documentation of proc macros to their re-export, such that
can link other items in frame-support. This is a patter that we should
embrace for all of macro docs, and apply in PRs like
https://github.com/paritytech/substrate/pull/13987 as well.

---------

Co-authored-by: Gonçalo Pestana <g6pestana@gmail.com>
Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: command-bot <>

substrate/frame/support/test/tests/pallet_ui/genesis_invalid_generic.stderr

@@ -1,4 +1,4 @@
-error: Invalid genesis builder: expected `GenesisBuild<T>` or `GenesisBuild<T, I>`
+error: Invalid genesis builder: expected `BuildGenesisConfig` (or the deprecated `GenesisBuild<T>` or `GenesisBuild<T, I>`)
   --> tests/pallet_ui/genesis_invalid_generic.rs:36:7
    |
 36 |     impl GenesisBuild for GenesisConfig {}