[GHC] #8888: Document Coercible in user's guide

[GHC]

#8888: Document Coercible in user's guide
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       Reporter:  goldfire          |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:  7.8.1
      Component:  Documentation     |           Version:  7.8.1-rc2
       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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 As far as I can tell, there is no discussion of `Coercible` in the User's
 Guide. Although the interface to `coerce` is essentially a vanilla API,
 the generation of `Coercible` instances is a language feature, and I
 believe deserves to be included.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8888>
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