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.
This sentence doesn't mean what you think it did. Its trying, poorly to say that there is not an event ordering between when the newListener event is emitted, and the listener is added to the array. The API is trying to formally weasly, allowing the EE implementation to choose whether to do do:

```
function addListener(event, listener) {
// emit('newListener', ... here?
this.listeners[event].push(listener)
// emit('newListener', ... here?
}
```

It would be much better for node to commit to doing either one, or the other, and document the event as occuring either "before" the listener is added or occuring "after" the listener is added, rather than telling people they should not rely on the order. Undefined event ordering is the bane of asynchronous development!