[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:  merge
        Priority:  normal            |        Milestone:  7.8.1
       Component:  Documentation     |          Version:  7.8.1-rc2
      Resolution:                    |         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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Comment (by simonpj):

 The example of `(~)` doesn't have one type constructor claiming to be
 defined in two different modules!  What you are doing here is totally new,
 and does not follow any previous pattern.

 To be specific: '''what goes wrong if you simply remove `Coercible` from
 `ghcPrimExports`'''?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8888#comment:12>
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