Document "Emitter" and its differences from Observer

[DavidMGross]

See e.g. ReactiveX/RxJava#4787

[RobLewis]

I too would very much appreciate a clearer explanation of Emitters. I'm a fairly experienced RxJava programmer (and native speaker of English) but TBH I can't understand basically any of this:

The Emitter is an abstraction over Observer that handles cancellation for you. In 1.x it extends Observer because that interface is just 3 methods. In 2.x the Observer has a 4th method, onSubscribe which you should not interact with but can't hide it if 2.x Emitter extended Observer. The sole purpose of Emitter is to let you interact with the downstream in a safe manner from within Observable.create.