Automatic merge from submit-queue

Adding clusters to the list of valid resources printed by kubectl help

Ref https://github.com/kubernetes/kubernetes/issues/25592

Adding clusters to the list of valid resources printed by kubectl help with a clear message that it only works when talking to federation apiserver.
In future, we should replace the hard coded list with a dynamic list generated using APIServer's discovery API.

```release-note
Adding clusters to the list of valid resources printed by kubectl help
```


cc @kubernetes/kubectl @kubernetes/sig-cluster-federation