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

[GHC]

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

 This reminded me I had that information in a text file for personal
 reference, I've placed it in LanguagePragmaHistory for everyone to see,
 which I guess may be helpful to whoever implements this task

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9665#comment:1>
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