[GHC] #12090: Document Weverything/Wall/Wextra/Wdefault in user's guide

[GHC]

#12090: Document Weverything/Wall/Wextra/Wdefault in user's guide
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           Reporter:  hvr            |             Owner:  hvr
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.0.2
          Component:  Documentation  |           Version:  8.0.1
           Keywords:  warnings       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:  Design/Warnings
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 The user's guide needs to be updated to reflect the feature implemented in
 phab:D1850

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12090>
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