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
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