Fixes and docs for usage as plugin

[nirs]

Using the new CommandDisplayNameAnnotation annotation introduced in Cobra 1.8.0.

Fixes help text issues with:

• help flag usage
• help command long description
• usage line for command without sub commands

[nirs]

Example runs using https://github.com/nirs/kubectl-cobraplugin

Root command help:

$ kubectl cobraplugin
Usage:
  kubectl cobraplugin [command]

Available Commands:
  completion  Generate the autocompletion script for the specified shell
  help        Help about any command
  subcmd      

Flags:
  -h, --help   help for kubectl cobraplugin

Use "kubectl cobraplugin [command] --help" for more information about a command.

Sub command help:

$ kubectl cobraplugin subcmd -h
Usage:
  kubectl cobraplugin subcmd [flags]

Flags:
  -h, --help   help for subcmd

Help command help:

$ kubectl cobraplugin help -h
Help provides help for any command in the application.
Simply type kubectl cobraplugin help [path to command] for full details.

Usage:
  kubectl cobraplugin help [command] [flags]

Flags:
  -h, --help   help for help

[nirs]

Examples run with sample-cli-plugin:
https://github.com/nirs/kubernetes/tree/sample-cli-plugin-help

$ kubectl ns -h
View or set the current namespace

Usage:
  kubectl ns [new-namespace] [flags]

Examples:

	# view the current namespace in your KUBECONFIG
	kubectl ns

	# view all of the namespaces in use by contexts in your KUBECONFIG
	kubectl ns --list

	# switch your current-context to one that contains the desired namespace
	kubectl ns foo


Flags:
  -h, --help                           help for kubectl ns
  ...

[nirs]

@jpmcb @marckhouzam did you have time to look at it?

[marckhouzam]

Thank you for following-up on these issues @nirs !
LGTM