[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:  andreas.abel      |                   Owner:  rasen
            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:  Phab:D1019
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Comment (by hvr):

 Fyi, the list in LanguagePragmaHistory is complete; so everything that
 7.10 supports but isn't mentioned in the list has been supported since
 before 7.0

 I didn't include the list of pragmas supported in 6.12, but here is the
 output of `ghc-6.12 --supported-languages`:

 https://gist.github.com/anonymous/5b28a41345e0a59644c2

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9665#comment:9>
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