Add API docs link if the CLI arg is a config class

[hmellor]

A QoL improvement for configs that are directly exposed to the CLI so that their API documentation is linked to.

$ vllm serve --help=VllmConfig
...
  --attention-config ATTENTION_CONFIG, -ac ATTENTION_CONFIG
                        Attention configuration.
                        API docs: https://docs.vllm.ai/en/latest/api/vllm/config/#vllm.config.AttentionConfig
                        Should either be a valid JSON string or JSON keys passed individually. (default: ...

[gemini-code-assist]

Code Review

The pull request introduces a quality-of-life improvement by adding API documentation links to CLI help messages for configuration classes. The implementation adds a new helper function _maybe_add_docs_url to generate these links and integrates it into the argument parsing logic. One area for improvement is the placement of an import statement within a function, which can lead to unnecessary overhead.