[GHC] #9358: Improve flag description in the user guide

[GHC]

#9358: Improve flag description in the user guide
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              Reporter:  jstolarek   |            Owner:  jstolarek
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:              |          Version:  7.8.3
  Documentation                      |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Easy (less than 1
  Unknown/Multiple                   |  hour)
       Type of failure:              |       Blocked By:
  Documentation bug                  |  Related Tickets:  #1880
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by carter):

 @jan,
 the -mavx2 and -mavx512* flags are currently unused in practice. They
 SHOULD NOT be documented in a user facing way till they actually do
 anything meaningful. Which they currently dont in practice.

 wrt -msse4.2, austin has generalized things so that GHC now has proper
 -march etc style flags afaict, the msse4.2 stuff should be there afaik

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9358#comment:18>
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