[GHC] #9665: Add "since" information to LANGUAGE extensions in GHC user guide

[GHC]

#9665: Add "since" information to LANGUAGE extensions in GHC user guide
-------------------------------------+-------------------------------------
        Reporter:  andreas.abel      |                   Owner:
            Type:  task              |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Documentation     |                 Version:  7.9
      Resolution:                    |                Keywords:  newcomer
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Changes (by thomie):

 * keywords:   => newcomer
 * priority:  high => normal


Comment:

 The file to change is `docs/users_guide/flags.xml`.

 Bonus: you'll learn something about all the language extensions GHC
 offers.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9665#comment:3>
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