usbtmc: document why the trigger re-arm result is ignored

A false return from tud_usbtmc_start_bus_read() here does not mean arming
failed: it means the endpoint is already armed, either because the
application re-armed it from its trigger callback or because a transfer is
still queued (usbd_edpt_xfer() reports failure when the endpoint is busy).
Both cases end in STATE_IDLE, so the state cannot disambiguate them either,
and stalling on the result would halt a healthy endpoint.

Co-Authored-By: Claude Opus 5 <[email protected]>

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