Missing samples in DOM events article

[jakub-g]

http://ariatemplates.com/usermanual/latest/dom_events

ctrl+f for "missing sample"

Also, in the last snippet, it should be emphasized that one should pass exactly the same references (or whole callback object references) to aria.utils.Event.addListener and removeListener (perhaps storing { fn : foo , scope: this } into this.myFooCallbackObj for convenience).