The watch.Interface design is hard to change, because it would break
most client-go users that perform watches. So instead of changing the
interface to be more user friendly, this change updates the method
comments to explain the different responsibilities of the consumer
(client user) and the producer (interface implementer).
Kubernetes-commit: 1f35231a1d4f7b8586a7ec589c799729eeb4f7c4