docs(observable): add a description for toPromise (#4960)
docs(observable): add a description for toPromise (#4960)

fix #4923
kwonoj/RxJS,ReactiveX/rxjs,martinsik/rxjs,kwonoj/RxJS,ReactiveX/RxJS,trxcllnt/RxJS,martinsik/rxjs,trxcllnt/RxJS,benlesh/RxJS,trxcllnt/RxJS,martinsik/rxjs,trxcllnt/RxJS,ReactiveX/RxJS,benlesh/RxJS,ReactiveX/rxjs,ReactiveX/RxJS,ReactiveX/rxjs,kwonoj/RxJS,benlesh/RxJS,ReactiveX/RxJS,ReactiveX/rxjs,martinsik/rxjs,kwonoj/RxJS,benlesh/RxJS