### Check for existing issues - [x] I have searched the existing issues and checked that this is not a duplicate.

### The Feature The `litellm_settings.include_cost_in_streaming_usage` setting (added in #13319) makes streaming responses include `usage.cost` in the final chunk when the request sets `stream_options.include_usage: true`. It works well, but it is not documented anywhere. Please add it to the cost tracking / streaming usage docs, with the config snippet and the note that it requires `stream_options.include_usage: true`.

```yaml litellm_settings: include_cost_in_streaming_usage: true ```

### Motivation, pitch The flag is not discoverable. The cost docs cover the `x-litellm-response-cost` header (non-streaming) but do not explain how to get cost on streaming, and there is no page for this setting. As a result people open requests for a capability that already ships (for example #30816). A short docs entry would close that gap. Origin is #13319.

### What part of LiteLLM is this about? Docs