[GHC] #9247: Document -XDatatypeContexts in flag reference

[GHC]

#9247: Document -XDatatypeContexts in flag reference
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       Reporter:  goldfire          |             Owner:  goldfire
           Type:  bug               |            Status:  new
       Priority:  low               |         Milestone:
      Component:  Documentation     |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 It may be a deprecated feature, but it still exists and should be listed.

 I can do this.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9247>
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