haddock discussion: Add option to hide definitions of given types

We intend to stop Cabal from passing -DHADDOCK_VERSION when compiling haddocks (see https://github.com/haskell/cabal/pull/9177). The most legitimate use of HADDOCK_VERSION out there is haskell-gi, which uses HADDOCK_VERSION to omit generating AttrLabelProxy top-level definitions for use with OverloadedLabels, such as in:

terminalAllowBold :: AttrLabelProxy "allowBold"

where

-- | A proxy for attribute labels.
data AttrLabelProxy (a :: Symbol) = AttrLabelProxy

instance a ~ x => IsLabel x (AttrLabelProxy a) where
    fromLabel = AttrLabelProxy

It is a bit controversial whether it's a good idea to omit certain definitions from the haddocks. I'm opening this ticket to track a discussion for a feature built-in to haddock to explicitly omit definitions of a certain type (in this case AttrLabelProxy).

@michaelpj suggests a per-definition {-# HADDOCK_OMIT #-} pragma which could hide any particular definition. I agree that this would be more principled and more useful than any type-directed solution. This fits the haskell-gi case too.