docs(delayWhen): Deprecate completion of notifier triggering source e…

…mission

This deprecates the behavior that the completion of the notifier observable will cause the source
emission to be emitted on the output observable.

src/internal/operators/delayWhen.ts

@@ -22,6 +22,8 @@ import { MonoTypeOperatorFunction, TeardownLogic } from '../types';
  * argument, and should return an Observable, called the "duration" Observable.
  * The source value is emitted on the output Observable only when the duration
  * Observable emits a value or completes.
+ * The completion of the notifier triggering the emission of the source value
+ * is deprecated behavior and will be removed in future versions.
  *
  * Optionally, `delayWhen` takes a second argument, `subscriptionDelay`, which
  * is an Observable. When `subscriptionDelay` emits its first value or
@@ -62,6 +64,10 @@ export function delayWhen<T>(delayDurationSelector: (value: T) => Observable<any
   return (source: Observable<T>) => source.lift(new DelayWhenOperator(delayDurationSelector));
 }
 
+/** @deprecated In future versions, empty notifiers will no longer re-emit the source value on the output observable. */
+export function delayWhen<T>(delayDurationSelector: (value: T) => Observable<never>,
+                             subscriptionDelay?: Observable<any>): MonoTypeOperatorFunction<T>;
+
 class DelayWhenOperator<T> implements Operator<T, T> {
   constructor(private delayDurationSelector: (value: T) => Observable<any>) {
   }