#6126: Fix risk of dead-lock in documentation of Control.Concurrent

#6126: Fix risk of dead-lock in documentation of Control.Concurrent
-------------------------------+--------------------------------------------
 Reporter:  basvandijk         |          Owner:                  
     Type:  bug                |         Status:  new             
 Priority:  normal             |      Component:  libraries/base  
  Version:  7.4.1              |       Keywords:                  
       Os:  Unknown/Multiple   |   Architecture:  Unknown/Multiple
  Failure:  Documentation bug  |       Testcase:                  
Blockedby:                     |       Blocking:                  
  Related:                     |  
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 The [http://hackage.haskell.org/packages/archive/base/4.5.0.0/doc/html
 /Control-Concurrent.html#g:12 current] documentation of
 `Control.Concurrent` proposes a function to wait for a child thread to
 complete:

 {{{
    myForkIO :: IO () -> IO (MVar ())
    myForkIO io = do
      mvar <- newEmptyMVar
      forkIO (io `finally` putMVar mvar ())
      return mvar
 }}}

 This function has the risk of causing a dead-lock. If an asynchronous
 exception if thrown before the `putMVar` exception handler is installed
 the `mvar` will remain empty. This causes a dead-lock when the `mvar` is
 taken.

 The attached patch fixes this problem by correctly masking asynchronous