Improve Collection docs

I've noticed a couple of people on the list confusing collection events with events on the objects in a collection, so thought this should be clarified.
that comment is an adaptation of what is currently in `ol.Object`. I'm not sure that having examples at this level is a good idea. I don't find the current bindTo example very helpful, for example in http://ol3js.org/en/master/apidoc/ol.layer.Image.html#bindTo.
I'd agree that it would be useful to have specific examples, but I think that's better in examples or in a separate tutorial rather than at the Object/Collection docs level. The problem is when I try and think of good uses for bindTo, I can't think of very many (though that may be more a problem with my limited imagination :-) ).