[GHC] #8190: mention --show-options in --help

[GHC]

#8190: mention --show-options in --help
-------------------------------------+------------------------------------
        Reporter:  carter            |            Owner:  thoughtpolice
            Type:  feature request   |           Status:  new
        Priority:  highest           |        Milestone:  7.8.1
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #7843
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Comment (by carter):

 @jan, on one hand you're right.

 On the other hand, most other compilers (eg clang and gcc) do have a way
 of listing all of the documented options, or at least most of them.

 however, Theres definitely value in having a curated list of all the
 stable flags when someone does
 {{{
 ghc -h
 or
 ghc --help
 }}}

 which we currently only do i the barest minimal sense possible

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8190#comment:5>
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