Add RxJava's fromEmitter() to the From operator page

[DavidMGross]

...assuming this gets merged in: ReactiveX/RxJava#4179

[DavidMGross]

See also: ReactiveX/RxJava#4253
(deprecation of create operator, rebatchRequests)

[DavidMGross]

(fromAsync was deprecated in favor of fromEmitter)

• 1.x: Rename Observable.fromAsync() RxJava#4255

[DavidMGross]

See also ReactiveX/RxJava#4618 (old RxJava wiki docs)