It is critical that Unsubscribe be called with the same pointer to a
SubscriptionRequest that was used to create the Subscription. The
docstring made that clear, but it sill allowed a caler to get it wrong by
creating a new SubscriptionRequest.
By hiding this detail from the caller, and only exposing an Unsubscribe
method, it should be impossible to fail to Unsubscribe.
Also update some godoc strings.
Daniel Nephin