refactor(mergeMap): slight efficiency improvement. Adds comments
refactor(mergeMap): slight efficiency improvement. Adds comments

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