QInputEvent: document that timestamp() is in milliseconds

Task-number: QTBUG-39459
Change-Id: Ie76f3c2f6fe81746d3ea646f0c2f0d47ec3ef252
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/gui/kernel/qevent.cpp

@@ -171,6 +171,8 @@ QInputEvent::~QInputEvent()
     \fn ulong QInputEvent::timestamp() const
 
     Returns the window system's timestamp for this event.
+    It will normally be in milliseconds since some arbitrary point
+    in time, such as the time when the system was started.
 */
 
 /*! \fn void QInputEvent::setTimestamp(ulong atimestamp)