docs(operators): improve switch() docs, and fix some typos
docs(operators): improve switch() docs, and fix some typos

Fix missing word, and add mention of unsubscriptions of older inner Observables.

ReactiveX/RxJS,staltz/RxJSNext,ReactiveX/rxjs,RxJS-CN/RxJS-Docs-CN,robwormald/RxJS,ReactiveX/rxjs,Brocco/RxJS,jayphelps/rxjs,Igorbek/rxjs5,Igorbek/rxjs5,martinsik/rxjs,elliotschi/rxjs,SangKa/RxJS-Docs-CN,figueredo/RxJS,SangKa/RxJS-Docs-CN,benlesh/RxJS,martinsik/rxjs,figueredo/RxJS,RxJS-CN/RxJS-Docs-CN,saneyuki/RxJS,martinsik/rxjs,robwormald/RxJS,saneyuki/RxJS,ReactiveX/RxJS,ReactiveX/rxjs,kwonoj/RxJS,RxJS-CN/RxJS-Docs-CN,Igorbek/rxjs5,SangKa/RxJS-Docs-CN,Igorbek/rxjs5,adrianomelo/RxJSNext,kwonoj/RxJS,adrianomelo/RxJSNext,kwonoj/RxJS,IgorMinar/RxJS,ReactiveX/RxJS,Brocco/RxJS,staltz/RxJSNext,benlesh/RxJS,trxcllnt/RxJS,staltz/RxJSNext,Brocco/RxJS,jayphelps/rxjs,martinsik/rxjs,gluck/RxJS,trxcllnt/RxJS,trxcllnt/RxJS,adrianomelo/RxJS-1,ReactiveX/rxjs,adrianomelo/RxJSNext,RxJS-CN/RxJS-Docs-CN,jayphelps/rxjs,adrianomelo/RxJS-1,kwonoj/RxJS,gluck/RxJS,elliotschi/rxjs,adrianomelo/RxJS-1,IgorMinar/RxJS,robwormald/RxJS,elliotschi/rxjs,benlesh/RxJS,ReactiveX/RxJS,gluck/RxJS,jayphelps/rxjs,figueredo/RxJS,benlesh/RxJS,elliotschi/rxjs,Brocco/RxJS,staltz/RxJSNext,figueredo/RxJS,gluck/RxJS,adrianomelo/RxJS-1,trxcllnt/RxJS,robwormald/RxJS,jayphelps/rxjs,SangKa/RxJS-Docs-CN,adrianomelo/RxJSNext