[C++20] [Coroutines] Add ReleaseNotes about addressing thread identifiaction problems

The thread identification problems in coroutines are addressed now in
1cedc51f. Although the original problem occurs in
LLVM optimizer. The C++ users have strong feeling about it. So it may be
necessary to add a ReleaseNote in clang for it.

Closes https://github.com/llvm/llvm-project/issues/47177
Closes https://github.com/llvm/llvm-project/issues/47179