Document the `--kubeconfig` flag, the `KUBECONFIG` env var, and in-cluster configs
Document the `--kubeconfig` flag, the `KUBECONFIG` env var, and in-cluster configs

Signed-off-by: Cornelius Weig <e24ffeec65acc52d9a334b9c7709d21fc4db45e8@users.noreply.github.com>

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