message: docs(toPromise): update toPromise documentation with undefined
  update documentation since toPromise signature was changed
  Closes
diff: @@ -354,13 +354,14 @@ export class Observable<T> implements Subscribable<T> {
  /**
  * Subscribe to this Observable and get a Promise resolving on
  - * `complete` with the last emission.
  + * `complete` with the last emission (if any).
  *
  * @method toPromise
  * @param [promiseCtor] a constructor function used to instantiate
  * the Promise
  * @return A Promise that resolves with the last value emit, or
  - * rejects on an error.
  + * rejects on an error. If there were no emissions, Promise
  + * resolves with undefined.
  */
  toPromise(promiseCtor?: PromiseConstructorLike): Promise<T | undefined> {
  promiseCtor = getPromiseCtor(promiseCtor);
  