[Misc] add description attribute in CLI

[reidliu41]

add description with -h:

from:
$ vllm chat -h
INFO 04-02 11:12:24 [__init__.py:239] Automatically detected platform cpu.
usage: vllm chat [options]

options:
  --api-key API_KEY     API key for OpenAI services. If provided, this api key will overwrite the
                        api key obtained through environment variables. (default: None)

to:
$ vllm chat -h
INFO 04-02 11:13:23 [__init__.py:239] Automatically detected platform cpu.
usage: vllm chat [options]

Generate chat completions via the running API server.    <<--

options:
  --api-key API_KEY     API key for OpenAI services. If provided, this api key will overwrite the
                        api key obtained through environment variables. (default: None)

[github-actions]

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🚀

[russellb]

thanks!