[GHC] #12365: Update documentation for partial type signatures

[GHC]

#12365: Update documentation for partial type signatures
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           Reporter:  goldfire       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Documentation  |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 In the [https://downloads.haskell.org/~ghc/master/users-
 guide/glasgow_exts.html#partial-type-signatures documentation] for partial
 type signatures, I found several bits that are out of date:

 - There are several sample error messages given. These have been
 substantively reworded ({{{Found hole `_` with type: ...}}} to {{{Found
 type wildcard `_` standing for ...}}}) where I find the new wording much
 clearer.

 - In the "Where can they occur?" section, there's no mention of
 `TypeApplications`, which is a new context where they can appear.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12365>
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