Xi: add a comment to make a condition a bit clearer

The commit message to 6764471901 explains it,
but that doesn't stop the WTF moment when reading the code.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Keith Packard <keithp@keithp.com>
(cherry picked from commit 2967391c6d)

Xi/exevents.c

@@ -1896,6 +1896,7 @@ DeliverTouchEndEvent(DeviceIntPtr dev, TouchPointInfoPtr ti, InternalEvent *ev,
         goto out;
     }
 
+    /* A client is waiting for the begin, don't give it a TouchEnd */
     if (listener->state == LISTENER_AWAITING_BEGIN) {
         listener->state = LISTENER_HAS_END;
         goto out;