Be explicit about bound threads in test execution in docs

dejafu/Test/DejaFu/Conc.hs

@@ -187,6 +187,12 @@ forkOSWithUnmaskN n ma
 -- state, returning a failure reason on error. Also returned is the
 -- final state of the scheduler, and an execution trace.
 --
+-- If the RTS supports bound threads (ghc -threaded when linking) then
+-- the main thread of the concurrent computation will be bound, and
+-- @forkOS@ / @forkOSN@ will work during execution.  If not, then the
+-- main thread will not be found, and attempting to fork a bound
+-- thread will raise an error.
+--
 -- __Warning:__ Blocking on the action of another thread in 'liftIO'
 -- cannot be detected! So if you perform some potentially blocking
 -- action in a 'liftIO' the entire collection of threads may deadlock!

doc/unit_testing.rst

@@ -17,7 +17,7 @@ with testcases which need ``MonadIO``, use ``dejafuIO`` for that.
 
 
 Actions
-----------
+-------
 
 An action is just something with the type ``MonadConc m => m a``, or
 ``(MonadConc m, MonadIO m) => m a`` for some ``a`` that your chosen
@@ -65,6 +65,13 @@ want to see the full code. [#]_
        predicates are checking that the bug is found, not that the
        code is correct.
 
+If the RTS supports bound threads (the ``-threaded`` flag was passed
+to GHC when linking), then the main thread of an action given to Déjà
+Fu will be bound, and further bound threads can be forked with the
+``forkOS`` functions.  If not, then attempting to fork a bound thread
+will raise an error.
+
+
 Predicates
 ----------