Merge pull request #30460 from juanvallejo/jvallejo_kube-add-zsh-compatibility-note-kubectl-completion

Automatic merge from submit-queue

Add zsh compatibility note `completion` cmd help

zsh completions are not supported on zsh versions < 5.2.

This patch advices user on supported versions of zsh when using the `completion`
command to avoid potential UX failure.

##### After
`$ kubectl completion -h`
```
Output shell completion code for the given shell (bash or zsh).

This command prints shell code which must be evaluation to provide interactive
completion of kubectl commands.

Examples:

$ source <(kubectl completion bash)

will load the kubectl completion code for bash. Note that this depends on the
bash-completion framework. It must be sourced before sourcing the kubectl
completion, e.g. on the Mac:

$ brew install bash-completion
$ source $(brew --prefix)/etc/bash_completion
$ source <(kubectl completion bash)

If you use zsh*, the following will load kubectl zsh completion:

$ source <(kubectl completion zsh)

* zsh completions are only supported in versions of zsh >= 5.2
```

```release-note
release-note-none
```

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