Document change in implicit quantification

Simon's big wildcard refactor (1e041b73) changed the way GHC does implicit quantification. I don't see documentation for this, however. (That patch doesn't touch the manual, and there are no release notes, at least.)

This report was prompted by this Haskell-Café post, where Tom Ellis asks why we need

   -data PackMap a b s t = PackMap (Applicative f => (a -> f b) -> s -> f t)
   +data PackMap a b s t = PackMap (forall f. Applicative f => (a -> f b) -> s -> f t)

to update for 8.0.

Trac metadata

Trac field	Value
Version	7.10.3
Type	Task
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Documentation
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