Haddocks for banned instances

[endgame]

Banning an instance makes it appear in the haddocks as an instance, so if you're not careful and miss the instance TypeError (...) => in the superclass, you get the opposite impression: that there is actually an instance that you can use.

I don't think TH can generate haddocks yet.

[endgame]

As of GHC 9.2.1, TH can generate haddock. It would be cool to have some CPP that does this where possible.