Adopt @philnik777's suggestion for release note

Co-authored-by: Nikolas Klauser <[email protected]>

Author: frederick-vs-ja

libcxx/docs/ReleaseNotes/21.rst

@@ -97,8 +97,7 @@ Potentially breaking changes
 - The implementation of ``num_put::do_put`` has been replaced to improve the performance, which can lead to different
   output when printing pointers.
 
-- User-defined specializations of ``std::common_reference`` are disallowed with the ``[[clang::no_specializations]]``
-  attribute. To customize the common reference type, ``std::basic_common_reference`` should be specialized instead.
+- User-defined specializations of ``std::common_reference`` are diagnosed now. To customize the common reference type, ``std::basic_common_reference`` should be specialized instead.
 
 Announcements About Future Releases
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