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

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

Author: ChuanqiXu9

clang/docs/ReleaseNotes.rst

@@ -706,6 +706,9 @@ Bug Fixes in This Version
   ``await_suspend`` could be misoptimized, including accesses to the awaiter
   object itself.
   (`#56301 <https://github.com/llvm/llvm-project/issues/56301>`_)
+  The current solution may bring performance regressions if the awaiters have
+  non-static data members. See
+  `#64945 <https://github.com/llvm/llvm-project/issues/64945>`_ for details.
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^