[GHC] #9957: Document -fwarn-unticked-promoted-constructors in release notes

[GHC]

#9957: Document -fwarn-unticked-promoted-constructors in release notes
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              Reporter:  adamgundry  |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:  7.10.1
             Component:              |           Version:  7.10.1-rc1
  Documentation                      |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  Documentation
          Architecture:              |  bug
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 The new flag `-fwarn-unticked-promoted-constructors` should be documented
 in the release notes, as users upgrading to 7.10 may need to update their
 code or flags to continue compiling with `-Wall -Werror`.

 Perhaps it should even be removed from `-Wall` (c.f.
 [https://www.haskell.org/pipermail/ghc-devs/2014-December/007738.html
 discussion here])?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9957>
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