Summary:
The section on `-XTypeApplications` in the users' guide isn't terribly
clear on how to view the visibility of a function type signature's type
variables in GHCi properly (i.e., using the `:type +v` GHCi command). This
adds some more exposition that demonstrates how to use `:type +v` (and why you
don't want to use `:type`).

Fixes #13401.

Test Plan: Eyeball it

Reviewers: bgamari, austin, goldfire, crockeea

Reviewed By: goldfire, crockeea

Subscribers: rwbarton, thomie, crockeea

Differential Revision: https://phabricator.haskell.org/D3310