rel-notes: Add note about UndecidableSuperClasses and Trac #11762
Test Plan: Read it
Reviewers: austin, kosmikus
Reviewed By: kosmikus
Subscribers: thomie
Differential Revision: https://phabricator.haskell.org/D2111
GHC Trac Issues: Trac #11318, Trac #11762
(cherry picked from commit 933abfa7ec88bd91e60a62e51bb2f9a068d379f1)