Test #9692 in th/T9692
Clarify location of Note. Comment change only.
Reify data family instances correctly.

Summary:
Fix #9692.

The reifier didn't account for the possibility that data/newtype
instances are sometimes eta-reduced. It now eta-expands as necessary.

Test Plan: th/T9692

Reviewers: simonpj, austin

Subscribers: thomie, carter, ezyang, simonmar

Differential Revision: https://phabricator.haskell.org/D355