[GHC] #15722: Add -Wstar-is-type to the User's Guide

[GHC]

#15722: Add -Wstar-is-type to the User's Guide
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           Reporter:  int-index      |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.6.2
          Component:  Documentation  |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):  Phab:D5203     |         Wiki Page:
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 Documentation for `-Wstar-is-type` is missing in the 8.6.1 release.

 Creating this ticket with a milestone as a reminder to cherry-pick the
 patch to 8.6.2 (right now it's merged to `master`).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15722>
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