Summary:
We mentioned that there were "some programs" that failed to typecheck
due to #12784, but given how surprisingly common this issue has been, it'd
be prudent to at least give one example of the bug in the release notes.

Reviewers: simonpj, bgamari, austin, rwbarton

Reviewed By: rwbarton

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2786

GHC Trac Issues: #12784