expand docs for forkOS

diff --git a/Control/Concurrent.hs b/Control/Concurrent.hs
index 1ce813c..45d2029 100644 (file)
--- a/Control/Concurrent.hs
+++ b/Control/Concurrent.hs
@@ -310,7 +310,11 @@ This means that you can use all kinds of foreign libraries from this thread
 
 Just to clarify, 'forkOS' is /only/ necessary if you need to associate
 a Haskell thread with a particular OS thread.  It is not necessary if
-you only need to make non-blocking foreign calls (see "Control.Concurrent#osthreads").
+you only need to make non-blocking foreign calls (see
+"Control.Concurrent#osthreads").  Neither is it necessary if you want
+to run threads in parallel on a multiprocessor: threads created with
+'forkIO' will be shared out amongst the running CPUs (using GHC,
+@-threaded@, and the @+RTS -N@ runtime option).
 
 -}
 forkOS :: IO () -> IO ThreadId