docs: drop the pr3840 no-boards handoff, both halves are done

The doc half landed in this branch. The test half it asked for already
exists: NoBoardsExitRespectsFreshness and TheNoBoardsCallSiteIsWired in
test/hil/test/test_hil_report.py (375e20090) cover the fresh and
--accumulate no-boards exits through mark_report_no_boards.