[GHC] #15266: Add QuantifiedConstraints to release notes

[GHC]

#15266: Add QuantifiedConstraints to release notes
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           Reporter:  goldfire       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Documentation
  Unknown/Multiple                   |  bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 It seems `-XQuantifiedConstraints` never made it into the release notes.
 We should fix this.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15266>
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