docs(offload): clarify LMCache MP lookup timeout scope

[kvnloo]

Summary

The earlier docs follow-up (#2407) was stacked on the native-MP feature branch and closed when that branch merged. Its remaining timeout clarification is not present on current main.

This one-file follow-up documents the control flow that still exists on main:

• lmcache.mp.mq_timeout belongs to LMCache adapter message-queue operations;
• lmcache.mp.lookup_timeout starts after lookup submission returns and bounds ATOM's polling loop;
• lmcache.mp.lookup_poll_interval controls sleeps between polls.

Therefore lookup_timeout is not a hard wall-clock bound around a blocking adapter submission or status call.

No runtime/default/topology behavior changes.

Evidence

The original deterministic CPU characterization is here:
https://github.com/kvnloo/ATOM/actions/runs/36160612338

That run exercised the real lookup facade with a fake adapter/clock and completed 65 MP tests. It demonstrated that submission time is outside the outer lookup polling deadline and that a completed status call is accepted before the next deadline check.

I re-read current main at 922b3519; _MPLookupClient.lookup() still submits first, sets the deadline afterward, then checks adapter status before checking the deadline.

No live server/GPU timeout behavior is claimed.

AI-assisted source review and drafting.

[github-actions]

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