rename metric for apiserver request terminations and reword corresponding documentation
rename metric for apiserver request terminations and reword corresponding documentation

Change-Id: I47a9c7b10614afe85bb652fa61984f91848d6d65

Kubernetes-commit: f8f1def5f1d92a588f48ebc01293e142f8dd63fd

kubernetes/kube-aggregator,kubernetes/kube-aggregator