Fix dot documentation

I goofed up and made the ``strat1 `dot` strat1 = strat1`` law too
strong. Fix that. Also, add laws for ``rpar `dot` rseq`` and
``rseq `dot` rpar``.

Control/Parallel/Strategies.hs

@@ -296,14 +296,18 @@ withStrategy = flip using
 -- | Compose two strategies sequentially.
 -- This is the analogue to function composition on strategies.
 --
--- For any strategies @strat1@, @strat2@, and @strat3@,
---
 -- > (strat1 `dot` strat2) `dot` strat3 == strat1 `dot` (strat2 `dot` strat3)
--- > strat1 `dot` strat1 = strat1
--- > strat1 `dot` r0 == strat1
+-- > strat `dot` r0 == strat
+-- > rpar `dot` rseq == rpar
+-- > rseq `dot` rpar == rseq
 --
 -- > strat2 `dot` strat1 == strat2 . withStrategy strat1
 --
+-- Furthermore, if @strat@ only forces and sparks computations (that is, it
+-- doesn't actually change any values), then
+--
+-- > strat `dot` strat == strat
+--
 dot :: Strategy a -> Strategy a -> Strategy a
 strat2 `dot` strat1 = strat2 . runEval . strat1