message: docs(retryWhen): Add example
  Added example: Trigger retry after specified duration.
diff: @@ -18,6 +18,43 @@ import { MonoTypeOperatorFunction, TeardownLogic } from '../types';
  *
  * ![](retryWhen.png)
  *
  + * ## Example
  + * ```ts
  + * import { timer, interval } from 'rxjs';
  + * import { map, tap, retryWhen, delayWhen } from 'rxjs/operators';
  + *
  + * const source = interval(1000);
  + * const example = source.pipe(
  + * map(val => {
  + * if (val > 5) {
  + * // error will be picked up by retryWhen
  + * throw val;
  + * }
  + * return val;
  + * }),
  + * retryWhen(errors =>
  + * errors.pipe(
  + * // log error message
  + * tap(val => console.log(`Value ${val} was too high!`)),
  + * // restart in 5 seconds
  + * delayWhen(val => timer(val * 1000))
  + * )
  + * )
  + * );
  + *
  + * const subscribe = example.subscribe(val => console.log(val));
  + *
  + * // results:
  + * // 0
  + * // 1
  + * // 2
  + * // 3
  + * // 4
  + * // 5
  + * // "Value 6 was too high!"
  + * // --Wait 5 seconds then repeat
  + * ```
  + *
  * @param {function(errors: Observable): Observable} notifier - Receives an Observable of notifications with which a
  * user can `complete` or `error`, aborting the retry.
  * @return {Observable} The source Observable modified with retry logic.
  