Update 8.0.2 release notes for #12784
Update 8.0.2 release notes for #12784

Summary:
The fix for #12220 exposed some ill-typed programs which passed the typechecker
in GHC 8.0.1 but now fail to typecheck in GHC 8.0.2. It's a bit difficult to
characterize what exactly triggers this bug, but we at least have a minimal
example and a simple fix to illustrate the problem and solution, so let's
add that the the 8.0.2 release notes to advertise this change.

Resolves #12784.

Reviewers: rwbarton, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2682

GHC Trac Issues: #12784

ezyang/ghc,olsner/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,mettekou/ghc,sdiehl/ghc,ezyang/ghc,olsner/ghc,mettekou/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,olsner/ghc,mettekou/ghc,shlevy/ghc,mettekou/ghc,sdiehl/ghc,olsner/ghc,mettekou/ghc,olsner/ghc,ezyang/ghc,shlevy/ghc,mettekou/ghc,ezyang/ghc,ezyang/ghc,olsner/ghc,mettekou/ghc,shlevy/ghc,olsner/ghc,shlevy/ghc