[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:
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Comment (by goldfire):

 Thanks, @rasen, for taking this on!

 It might also be helpful in your work to note major changes made to
 extensions, such as the fact that 7.8's `TypeFamilies` includes closed
 type families, while previously the extension worked only for open ones.
 (There was actually some debate at the time about whether we should create
 a new extension.) Tracking these changes may be challenging, though --
 don't let this request hold you up from getting a baseline "since x.x.x"
 working.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9665#comment:5>
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