[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:
                  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:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by jstolarek):

 `-frule-check` flag is not described at all. Here's an excerpt from
 `Rules.lhs`:

 {{{
    We want to know what sites have rules that could have fired but didn't.
    This pass runs over the tree (without changing it) and reports such.

 \begin{code}
 -- | Report partial matches for rules beginning with the specified
 -- string for the purposes of error reporting
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9358#comment:7>
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