message: docs(mapTo): add comments
diff: @@ -44,6 +44,13 @@ export function mapTo<R>(value: R): OperatorFunction<any, R> {
  return (source: Observable<any>) =>
  lift(source, function (this: Subscriber<R>, source: Observable<any>) {
  const subscriber = this;
  - source.subscribe(new OperatorSubscriber(subscriber, () => subscriber.next(value)));
  + // Subscribe to the source. All errors and completions are forwarded to the consumer
  + source.subscribe(
  + new OperatorSubscriber(
  + subscriber,
  + // On every value from the source, send the `mapTo` value to the consumer.
  + () => subscriber.next(value)
  + )
  + );
  });
  }
  