Fix ValueObservation duration - code & doc

[groue]

This PR fixes a bug in ValueObservation, where unwanted notifications could happen despite deallocation of the observer returned by the ValueObservation.start method.

var observer: TransactionObserver? = valueObservation.start(in: dbQueue) { _ in
    print ("database has changed")
}
...
// fixed: now guarantees that "database has changed" won't be printed
// past this point.
observer = nil

Documentation has also been updated in order to warn about unsafe ways to control the duration of observation.