[red-knot] Document all lints

[MichaReiser]

With #14873, it's now required to document every lint. The PR itself documented some lints but not all of them.

The goal of this issue is to go through all lints and add documentation to them, similar to what we have in ruff.

[InSyncWithFoo]

Is this help-wanted? There are currently 21 rules with /// TODO #14889; that should only take up one of my afternoons or so.

[MichaReiser]

Thanks for asking. IMO, it has fairly low priority at the moment and we don't even know if we'll keep the rules as they are right now. That's why I probably would wait.