Document new Single.doOnFoo() operators

[DavidMGross]

See:

• Add Single.doOnSuccess() RxJava#3417 doOnSuccess( )
• Add Single.doOnError() RxJava#3419 doOnError( )
• Add Single.doOnUnsubscribe() RxJava#3562 doOnUnsubscribe( )
• Add doOnSubscribe for Single RxJava#3741 doOnSubscribe( )

Also, keep an eye on:

• Add Single.finallyDo() RxJava#3434 doAfterTerminate()

Currently not found at http://reactivex.io/documentation/single.html