[NFC] [C++20] [Coroutines] Mention the side effect of a fix may bring regressions

The fix we sent for https://github.com/llvm/llvm-project/issues/56301
may bring performance regressions. But we didn't mention it in the
ReleaseNotes so that users may get confused. e.g,
https://github.com/llvm/llvm-project/issues/64933. So this patch
mentions the possible side effect and the potential solutions in
https://github.com/llvm/llvm-project/issues/64945 to avoid
misunderstandings.