Give concrete example for Trac #12784 in 8.0.2 release notes
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
Differential Revision: https://phabricator.haskell.org/D2786
GHC Trac Issues: Trac #12784