[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 jstolarek):

 Note to self: `-msse4.2` mentioned in the section 4.16 seems to have been
 removed. There are also some new flags added by Geoffrey: `-mavx2`,
 `-mavx512cd`, `-mavx512er`, `-mavx512f` and `-mavx512pf`. I'm unable to
 document these, need to ask Geoffrey.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9358#comment:17>
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