Summary:
I recently (re-)discovered that `ForallC` quantifies different type variables
depending on whether `GadtC` is present or not. This is an important
enough gotcha where I feel like this fact should also be advertised in the
`template-haskell` documentation itself, so this patch does just that.

Test Plan: Read it

Reviewers: goldfire, austin, bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #13885

Differential Revision: https://phabricator.haskell.org/D3880