message: docs(timestamp): add short-description, description and example
diff: @@ -4,6 +4,34 @@ import { OperatorFunction, SchedulerLike, Timestamp as TimestampInterface } from
  import { map } from './map';
  /**
  + * Attaches a timestamp to each item emitted by an observable indicating when it was emitted
  + *
  + * The `timestamp` operator maps the *source* observable stream to an object of type
  + * `{value: T, timestamp: R}`. The properties are generically typed. The `value` property contains the value
  + * and type of the *source* observable. The `timestamp` is generated by the schedulers `now` function. By
  + * default it uses the *async* scheduler which simply returns `Date.now()` (milliseconds since 1970/01/01
  + * 00:00:00:000) and therefore is of type `number`.
  + *
  + * ![](timestamp.png)
  + *
  + * ## Example
  + *
  + * In this example there is a timestamp attached to the documents click event.
  + *
  + * ```ts
  + * import { fromEvent } from 'rxjs';
  + * import { timestamp } from 'rxjs/operators';
  + *
  + * const clickWithTimestamp = fromEvent(document, 'click').pipe(
  + * timestamp()
  + * );
  + *
  + * // Emits data of type {value: MouseEvent, timestamp: number}
  + * clickWithTimestamp.subscribe(data => {
  + * console.log(data);
  + * });
  + * ```
  + *
  * @param scheduler
  * @return {Observable<Timestamp<any>>|WebSocketSubject<T>|Observable<T>}
  * @method timestamp
  