[GHC] #12461: Document -O3

[GHC]

#12461: Document -O3
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        Reporter:  crockeea          |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Documentation     |  Unknown/Multiple
  bug                                |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mpickering):

 I doesn't do anything. Anything higher than 2 is set to 2. The relevant
 function is `DynFlags.updOptLevel`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12461#comment:1>
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