Add description to delete cluster subcommands

[stmcginnis]

This adds a more detailed description to the kind delete cluster and kind delete clusters commands explaining expected delete behavior.

Hopefully this reduces some of the regular confusion from users submitting issues due to expecting an error deleting a cluster that does not exist.

Updated command output:

$ kind delete cluster --help
Deletes a Kind cluster from the system.

This is an idempotent operation, meaning it may be called multiple times without
failing. If the cluster exists it will be deleted, and if the cluster is already
gone it will just return success. Errors will only occur if the cluster exists
and was not able to be deleted.

Usage:
  kind delete cluster [flags]

Flags:
  -h, --help                help for cluster
      --kubeconfig string   sets kubeconfig path instead of $KUBECONFIG or $HOME/.kube/config
  -n, --name string         the cluster name (default "kind")

Global Flags:
  -q, --quiet             silence all stderr output
  -v, --verbosity int32   info log verbosity, higher value produces more output

[BenTheElder]

thanks! some suggestions + gofmt is required

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: stmcginnis
Once this PR has been reviewed and has the lgtm label, please assign bentheelder for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment