base: Fix documentation for forkIOWithUnmask

forkIOUnmasked has been deprecated for several years now. Update reference to
it. See #4858 and #5546.

(cherry picked from commit 18401210)

libraries/base/GHC/IO.hs

@@ -335,7 +335,7 @@ onException io what = io `catchException` \e -> do _ <- what
 -- use @mask_ $ forkIO ...@.  This is particularly useful if you need
 -- to establish an exception handler in the forked thread before any
 -- asynchronous exceptions are received.  To create a a new thread in
+-- an unmasked state use 'Control.Concurrent.forkIOWithUnmask'.
 --
 mask  :: ((forall a. IO a -> IO a) -> IO b) -> IO b