docs(observeOn): create the documentation (#2541)

Create documentation for the operator, describing
what it can be used for, what parameters accepts and returns.
Add example of typical usage and compare the operator with delay
operator.

src/operator/observeOn.ts

@@ -8,11 +8,48 @@ import { TeardownLogic } from '../Subscription';
 import { Action } from '../scheduler/Action';
 
 /**
- * @see {@link Notification}
  *
- * @param scheduler
- * @param delay
- * @return {Observable<R>|WebSocketSubject<T>|Observable<T>}
+ * Re-emits all notifications from source Observable with specified scheduler.
+ *
+ * <span class="informal">Ensure a specific scheduler is used, from outside of an Observable.</span>
+ *
+ * `observeOn` is an operator that accepts a scheduler as a first parameter, which will be used to reschedule
+ * notifications emitted by the source Observable. It might be useful, if you do not have control over
+ * internal scheduler of a given Observable, but want to control when its values are emitted nevertheless.
+ *
+ * Returned Observable emits the same notifications (nexted values, complete and error events) as the source Observable,
+ * but rescheduled with provided scheduler. Note that this doesn't mean that source Observables internal
+ * scheduler will be replaced in any way. Original scheduler still will be used, but when the source Observable emits
+ * notification, it will be immediately scheduled again - this time with scheduler passed to `observeOn`.
+ * An anti-pattern would be calling `observeOn` on Observable that emits lots of values synchronously, to split
+ * that emissions into asynchronous chunks. For this to happen, scheduler would have to be passed into the source
+ * Observable directly (usually into the operator that creates it). `observeOn` simply delays notifications a
+ * little bit more, to ensure that they are emitted at expected moments.
+ *
+ * As a matter of fact, `observeOn` accepts second parameter, which specifies in milliseconds with what delay notifications
+ * will be emitted. The main difference between {@link delay} operator and `observeOn` is that `observeOn`
+ * will delay all notifications - including error notifications - while `delay` will pass through error
+ * from source Observable immediately when it is emitted. In general it is highly recommended to use `delay` operator
+ * for any kind of delaying of values in the stream, while using `observeOn` to specify which scheduler should be used
+ * for notification emissions in general.
+ *
+ * @example <caption>Ensure values in subscribe are called just before browser repaint.</caption>
+ * const intervals = Rx.Observable.interval(10); // Intervals are scheduled
+ * // with async scheduler by default...
+ *
+ * intervals
+ * .observeOn(Rx.Scheduler.animationFrame) // ...but we will observe on animationFrame
+ * .subscribe(val => { // scheduler to ensure smooth animation.
+ * someDiv.style.height = val + 'px';
+ * });
+ *
+ * @see {@link delay}
+ *
+ * @param {IScheduler} scheduler Scheduler that will be used to reschedule notifications from source Observable.
+ * @param {number} [delay] Number of milliseconds that states with what delay every notification should be rescheduled.
+ * @return {Observable<T>} Observable that emits the same notifications as the source Observable,
+ * but with provided scheduler.
+ *
  * @method observeOn
  * @owner Observable
  */