Document apply_function dot notation requirement for Kafka message queues

[aeroyorch]

Add some docs about apply_function dot notation requirement for Kafka message queues.

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Was generative AI tooling used to co-author this PR?

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Claude Opus 4.8 for inspecting existing docs.

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✅ Ready for review · @aeroyorch → @potiuk · 2026-06-12 13:30 UTC

Thanks @aeroyorch — all checks are green and this PR is marked ready for maintainer review. The ball is with the maintainers now; a maintainer will take the next look.

_{Automated triage — may be imperfect.}