add note that support already existed before as an extension

Co-authored-by: Mariya Podchishchaeva <[email protected]>

Author: h-vetinari

clang/docs/ReleaseNotes.rst

@@ -318,7 +318,7 @@ C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^
 
 - Clang now supports `N3029 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3029.htm>`_ Improved Normal Enumerations.
-- Clang now supports `N3030 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3030.htm>`_ Enhancements to Enumerations.
+- Clang now officially supports `N3030 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3030.htm>`_ Enhancements to Enumerations. Clang already supported it as an extension, so there were no changes to compiler behavior.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------