Add comment about negative time

Specs/Scene/SceneSpec.js

@@ -1098,7 +1098,7 @@ defineSuite([
 
         var spyListener = jasmine.createSpy('listener');
         s.camera.moveEnd.addEventListener(spyListener);
-
+        // We use negative time here to ensure the event runs on the next frame.
         s.cameraEventWaitTime = -1.0;
         s.camera.moveLeft();
         // The first render will trigger the moveStart event.