Give concrete example for #12784 in 8.0.2 release notes

Authored by RyanGlScott on Dec 6 2016, 8:03 AM.

Description

Give concrete example for Trac #12784 in 8.0.2 release notes

Summary:
We mentioned that there were "some programs" that failed to typecheck
due to Trac #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: Trac #12784

(cherry picked from commit eec02ab7c8433465cc8d6be0a8889e7c6a222fb0)