observe_field does not use :on option

This isn't a rails bug, really. The ability for prototype's Form.Element.EventObserver to observe specific events was actually removed over 2 and a half years ago, in this commit. Hard to tell whether this was intentional.

Anyway if you want it back, I suggest you take it up with the prototype folks. In the mean time we should probably remove all mention of :on from the code/docs as it's just confusing people

I don't need this feature really anymore, so I would suggest as well to delete it from the rails documentation as it was really confusing.