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.

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Co-authored-by: Gonçalo Pestana <[email protected]>
Co-authored-by: joe petrowski <[email protected]>
Co-authored-by: command-bot <>