The prior comment claimed anyio.lowlevel.checkpoint() yields None and is
re-driven, but checkpoint() yields a non-None backend object (asyncio
Future / trio checkpoint) and so trips the RuntimeError guard. Clarify
that only a literal bare yield/yield None is re-driven; any real awaitable
is caught by the non-None guard. Comment-only; no behavior change.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>