Events doc fixes

Documentation fix for #8853 ... specifically, acknowledge in the docs that EE.addListener(...) does not check to see if the given listener has already been added, and that newListener will be called regardless of whether an already existing listener is added twice.
Perhaps... 

>This event is emitted any time someone adds a listener. When the event is triggered, the listener may not yet have been added to the list returned by `emitter.listeners(event)`. 
>If any single listener happens to be added multiple times (see `emitter.addListener`), the `newListener` event will trigger once for each time it is added. 

Truthfully, tho, the best solution would be to simply prevent listeners from being added multiple times.