reword comment about order of adding waitables

The old comment only mentioned times, but this applies to the other waitables as well.

rcldotnet/RCLdotnet.cs

@@ -1221,14 +1221,17 @@ public static void SpinOnce(Node node, long timeout)
                     WaitSetAddGuardCondition(waitSetHandle, guardCondition.Handle);
                 }
 
-                // Add timers to WaitSet before action clients and servers.
-                // As ActionClient and ActionServer also register timers internally,
-                // the order of adding them to the WaitSet has to match the execution order 
                 foreach (var timer in node.Timers)
                 {
                     WaitSetAddTimer(waitSetHandle, timer.Handle);
                 }
 
+                // Action clients and servers need to be registerd after all the
+                // others that are handled by
+                // RCLdotnetDelegates.native_rcl_wait_set_*_ready methods, as
+                // they track indexes of the given waitables. Adding action
+                // clients and action servers before will get those indexes
+                // mixed up.
                 foreach (var actionClient in node.ActionClients)
                 {
                     WaitSetAddActionClient(waitSetHandle, actionClient.Handle);