Document new Observable.fromCallable() operator

[DavidMGross]

There was already an operator by that name in the rxjava-async package (see http://reactivex.io/documentation/operators/start.html) but now it's in Observable.java as well. Not sure if we should continue to document the rxjava-async version or just remove the rxjava-async disclaimer from the description of that operator. See also the mention at http://reactivex.io/documentation/operators/from.html

See:

• Add Observable.fromCallable() as a companion for Observable.defer() RxJava#3154
• Deprecate Async.fromCallable in favor Observable.fromCallable RxJavaAsyncUtil#7