Update throttle comment

src/Control/Concurrent/Throttle.hs

@@ -34,7 +34,13 @@ action only as frequently as allowed and other calls are DROPPED. This
 is in contrast to things like Conduit throttling, where actions are
 just SLOWED DOWN. We use this for asynchronous notifications and, if
 messages are the same, we can just drop them safely. Our input is the
-provided 'TChan.TChan'.  This function should be spawned as a thread.
+provided 'TChan.TChan'.
+
+This function should be spawned as a thread.
+
+We provide separate 'id' and 'a'. 'id' is used for uniquely
+identifying groups of throttled messages, while 'a' are actual
+messages that are sent to 'action' callback.
 -}
 throttle :: (Ord id, Eq id, Show id) => Int -> TChan.TChan (id, a) -> (a -> IO ()) -> IO ()
 throttle delay tchan action = do