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