Improve descriptions of our error codes (#450)

This PR tries to make the motivation for each flake8-pyi rule clear. As
ruff has reimplemented flake8-pyi's error codes, it's come up a number
of times that it isn't always 100% clear why we're enforcing a
particular rule: some of our error codes enforce things to do with
correctness, but some of them are purely stylistic. (See
astral-sh/ruff#8765 for a recent example.)
I've tried to divide our error codes into four broad categories to
rectify this.

Several of our rules are specifically to do with things working
differently in a stub file compared to at runtime. For some of these
rules, I've also tried to add a few more words to explain exactly how
stubs differ from `.py` files.