Updated haddocks for GHC 8.6 compatibility.

[kquick]

No description provided.

[eddywestbrook]

Hey @kquick, I never merged this, sorry; does this still need to be merged?

[kquick]

Probably, @eddywestbrook but I don't have time to update it for the conflicts until next week. The main issue is that Haddock doesn't support post-documentation of GADT constructors, only pre-documentation.

Bad:

    data MyGADT where
        Con :: ...
        - ^ This is doc for Con

Good:

    data MyGADT where
        - | This is doc for Con
        Con :: ...

These changes are pretty mechanical.