Document error codes

[JukkaL]

Follow-up to #7239.

I'm thinking of adding non-trivial documentation for each error code, as this would also serve as documentation about the kinds of issues mypy can find.

[ilevkivskyi]

Probably this was already discussed elsewhere, but maybe we should add a command flag like --list-error-codes that will just print a simple table like error-code: Short description like in errorcodes.py?

[JukkaL]

@ilevkivskyi Yeah, that was the idea. I'll create an issue.

[JukkaL]

This was implemented in #7451.