SharedInformerFactory starts goroutines in Start and those can be stopped by
closing the stop channel. However, there was no API that waits for the
goroutines.
This is a problem for unit testing. A test has to return while the informers
are still running, which may get flagged by tools like
https://github.com/uber-go/goleak or by klog/ktesting when those informers
lead to log output.
While at it, more documentation gets added to address
https://github.com/kubernetes/kubernetes/issues/65036.
Kubernetes-commit: e89d1d47e8365cad31600b17dab662d3c8a359dd
