Complete bi-directional linking between cli-flags/configs and error codes

Could there also be links (both ways) between those and the documentation of the different error codes that are affected by the configuration options (eg. between --disallow-any-generics, disallow_any_generics and type-arg). In this example, there is also the "Generics" section of the documentation.

Some of those links are already there, some are not there yet.

Originally posted by @Zeckie in #12347

I agree with Zeckie, having just spent some time looking at --warn-unreachable and thinking "hmm, wasn't this some kind of code as well?" and then remembering [unreachable]

Possibly, flags which could be replaced with a simple --enable-error-code whatever should be deprecated in some way. (Although, it probably doesn't make sense to remove them, since they're so cheap to maintain.) But that would be a different issue, anyway.

This bi-directional linking could probably be achieved for free an automatically if more of the docs were automatically generated, a perennial thought of mine.

If the other issue merging config and cli guides is fixed, then this issue would naturally become easier, or perhaps get fixed along the way.

Start with the command_line.html and error_code_list2.html documentation pages, following the existing links for flags such as --warn-unreachable and error codes such as unreachable. Compare the affected CLI flags, configuration names, error codes, and related documentation sections, then ensure the relevant references work in both directions.