docs(timestamp): add short-description, description and example (#4699)

#4615

src/internal/operators/timestamp.ts

@@ -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