doc: clarify QUIC async write backpressure

[johnfinnerty-nz]

Summary

Document strict async write behavior, ERR_INVALID_STATE, canWrite, and onblocked.

Validation

Documentation was checked against the current implementation; git diff --check passed.

Closes #63305

[nodejs-github-bot]

Review requested:

• @nodejs/quic

[github-actions]

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[jasnell]

@johnfinnerty-nz ... While the change here looks fine and is acceptable, if you make future additional contributions, please do ensure that the PR description is not just AI generated. We need to know that the contribution comes from a person, not an AI agent.

[johnfinnerty-nz]

Cheers James. I did use Codex, including to polish my description, as grammar isn’t my strong suit. I reviewed the code before submitting. The change clarifies that asynchronous QUIC writes reject with ERR_INVALID_STATE when the buffer is full, rather than waiting for capacity. I’ll make sure future descriptions explain the reasoning in my own words. Thanks again for the review and feedback.