Add release note

Author: frederick-vs-ja

libcxx/docs/ReleaseNotes/21.rst

@@ -97,6 +97,9 @@ 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.
+
 Announcements About Future Releases
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