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.
1. event api is frozen
2. its a feature
3. if you don't like it... you can always do a wrapper, call listeners(event), see if your listener is in the array, and then don't add it if you don't want to.

So, you can get the no-multiples behaviour from the current API, whereas if it didn't allow multiples, you couldn't get the current behaviour.