[GHC] #8502: Document a surprising {{{unsafeDupablePerformIO}}} limitation.

[GHC]

#8502: Document a surprising {{{unsafeDupablePerformIO}}} limitation.
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        Reporter:  int-e             |            Owner:
            Type:  task              |           Status:  new
        Priority:  normal            |        Milestone:  7.8.1
       Component:  libraries/base    |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  Documentation     |       Difficulty:  Easy (less than 1
  bug                                |  hour)
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by int-e):

 The same limitation applies to {{{unsafeIOToST}}}, which is currently
 completely undocumented.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8502#comment:1>
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