Merge pull request #7821 from cakephp/5.next-enum

Docs for enum additions.

Author: markstory

en/appendices/5-1-migration-guide.rst

@@ -92,6 +92,8 @@ Validation
 
 - ``Validation::enum()`` and ``Validator::enum()`` were added. These validation
   methods simplify validating backed enum values.
+- ``Validation::enumOnly()`` and ``Validation::enumExcept()`` were added to check for specific cases
+  and further simplify validating backed enum values.
 
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