Register a4a1996 in the CHANGELOG

References rails#55089.

Author: fxn

activerecord/CHANGELOG.md

@@ -1,3 +1,13 @@
+*   `:class_name` is now invalid in polymorphic `belongs_to` associations.
+
+    Reason is `:class_name` does not make sense in those associations because
+    the class name of target records is dynamic and stored in the type column.
+
+    Existing polymorphic associations setting this option can just delete it.
+    While it did not raise, it had no effect anyway.
+
+    *Xavier Noria*
+
 *   Add support for multiple databases to `db:migrate:reset`.
 
     *Joé Dupuis*