[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:  8.0.3
       Component:  Documentation     |              Version:  8.0.1
      Resolution:                    |             Keywords:  warnings
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:  Design/Warnings   |
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Comment (by phadej):

 Does `-Wextra` include every other `-W...` flag?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12090#comment:3>
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