Per @sxyazi's feedback on #4140: the previous wording ("if an AI
agent opened this PR") implied AI could act as the one submitting
the PR, which conflicts with the AI Policy requirement that PR
descriptions are written by humans. Reworded so the note tells the
human contributor to write and post the description themselves.
Suggested by @sxyazi in response to a comment on #4137: a quick
reminder near the AI Policy checkbox for contributors whose PR was
opened by an AI agent, since automated submission can occasionally
retry and leave behind an incomplete duplicate PR.
