Marking typeclass instances “worth of note”

[Profpatsch]

https://twitter.com/Profpatsch/status/1467790991763587074

Haddock should have a way of marking typeclass instances as “worth of note”, putting them in their own section.

Example: Last has two instances users use the class for, Semigroup (Last a) and Monoid (Last a), the rest is “plumbing”.