message: docs(single): Describe case where `undefined` is emitted
  Closes
diff: @@ -8,7 +8,8 @@ import { Observer, MonoTypeOperatorFunction, TeardownLogic } from '../types';
  /**
  * Returns an Observable that emits the single item emitted by the source Observable that matches a specified
  * predicate, if that Observable emits one such item. If the source Observable emits more than one such item or no
  - * such items, notify of an IllegalArgumentException or NoSuchElementException respectively.
  + * items, notify of an IllegalArgumentException or NoSuchElementException respectively. If the source Observable
  + * emits items but none match the specified predicate then `undefined` is emiited.
  *
  * <img src="./img/single.png" width="100%">
  *
  @@ -16,8 +17,8 @@ import { Observer, MonoTypeOperatorFunction, TeardownLogic } from '../types';
  * callback if the Observable completes before any `next` notification was sent.
  * @param {Function} predicate - A predicate function to evaluate items emitted by the source Observable.
  * @return {Observable<T>} An Observable that emits the single item emitted by the source Observable that matches
  - * the predicate.
  - .
  + * the predicate or `undefined` when no items match.
  + *
  * @method single
  * @owner Observable
  */
  