[GHC] #15228: Remove use of showSDocUnsafe in extending_ghc.rst

[GHC]

#15228: Remove use of showSDocUnsafe in extending_ghc.rst
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           Reporter:  mpickering     |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:  newcomer       |  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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 There are lots of usages of `showSDocUnsafe` in `extending_ghc.rst`.
 However, they are all unnecessary as far as I could see because the `Hsc`
 or `TcM` context has a copy of `DynFlags` in it. It should be possible to
 replace them all with `showSDoc`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15228>
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