Observer: Document the ObserverTracker class being returned

Currently the documentation only mentions this class in passing and with no useful detail:
> Returns an instance of the ObserverTracker which allows users to have manual control over disconnecting the mutationObserver and otherwise track its status.

and-

> @returns {ObserverTracker}

There's also no mention in the example. Users shouldn't need to view the code to learn about class members.
