Improve documentation of doOnUnsubscribe( )

[DavidMGross]

Clarify whether doOnUnsubscribe( ) calls its action when it unsubscribes from the source Observable, not when its subscribers unsubscribe from it's resultant Observable; or whether it calls its action whenever observers to its resultant Observable unsubscribe from it... there's some confusion on the point. The marble diagram may need to be made more evocative of this as well.

See: ReactiveX/RxJava#3877

[DavidMGross]

see ReactiveX/RxJava#3907

• https://github.com/ReactiveX/RxJava/pull/3907/files#diff-0
• http://reactivex.io/documentation/operators/do.html