fix documentation of doOnEach( )

[DavidMGross]

apparently it triggers its observer/action on both emissions and notifications (error/complete); currently the docs (and diagrams) say that it only triggers on emissions, like doOnNext().

See ReactiveX/RxJava#3721

[DavidMGross]

It's probably worth an explanation somewhere of the subtle difference between subscribe() and doOnEach() -- maybe both in the doOnEach operator page and in the page that describes the observable/observer and subscription process.

[akarnokd]

We decided not to have this operator for now, can be closed.

[DavidMGross]

The text at http://reactivex.io/documentation/operators/do.html still needs to be changed to reflect that doOnEach emits for items & notifications while doOnNext only emits for items.