docs: note OpenAI client base_url for multi-model gateways

[seven7763]

Summary

OpenAIChatGenerator already documents custom deployments via api_base_url.

This PR adds a short tip that the same parameter works with OpenAI-compatible multi-model gateways, using DaoXE (https://api.daoxe.com/v1) as one concrete example.

Docs only — no runtime behavior changes.

Test plan

• Docs page renders
• Existing api_base_url sentence unchanged in meaning
• Tip is clearly optional

Co-Authored-By: Claude Fable 5 noreply@anthropic.com

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