[GHC] #5124: Improve documentation for Data.Traversable.fmapDefault

[GHC]

#5124: Improve documentation for Data.Traversable.fmapDefault
---------------------------------+------------------------------------------
    Reporter:  byorgey           |       Owner:                   
        Type:  bug               |      Status:  new              
    Priority:  normal            |   Component:  libraries/base   
     Version:  7.0.3             |    Keywords:                   
    Testcase:                    |   Blockedby:                   
          Os:  Unknown/Multiple  |    Blocking:                   
Architecture:  Unknown/Multiple  |     Failure:  Documentation bug
---------------------------------+------------------------------------------
 Data.Traversable.fmapDefault in fact only works with Traversable instances
 which override the definition of traverse, not with instances which only
 override sequenceA (although such instances are perfectly valid).  I've
 attached a patch noting this in the fmapDefault documentation.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5124>
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