docs(retryWhen): Add description #4626
docs(retryWhen): Add description #4626

Added a simple description about for what the operator is useful.

martinsik/rxjs,martinsik/rxjs,martinsik/rxjs,benlesh/RxJS,ReactiveX/rxjs,ReactiveX/RxJS,benlesh/RxJS,trxcllnt/RxJS,ReactiveX/rxjs,kwonoj/RxJS,kwonoj/RxJS,ReactiveX/rxjs,ReactiveX/RxJS,kwonoj/RxJS,ReactiveX/RxJS,trxcllnt/RxJS,trxcllnt/RxJS,benlesh/RxJS,kwonoj/RxJS,benlesh/RxJS,trxcllnt/RxJS,ReactiveX/rxjs,martinsik/rxjs,ReactiveX/RxJS