[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:
           Type:  task               |                  Status:  new
       Priority:  high               |               Milestone:
      Component:  Documentation      |                 Version:  7.9
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 If one wants to support older versions of GHC, it is hard to find out
 whether one can use a certain LANGUAGE extension. One has to go to older
 versions of the GHC manual to find out whether the corresponding
 documentation was already there, or browse release notes.  Requests:

 1. Document for new LANGUAGE extensions that they are available since
 version 7.10.  For example, by adding "(Since GHC 7.10)" to the heading of
 the new feature.

 2. Do this also for the previous LANGUAGE extensions, by adding e.g.,
 "(Since GHC 7.6)" etc.

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