add HHH-19614 to migration guide

Author: gavinking

migration-guide.adoc

@@ -22,7 +22,19 @@ earlier versions, see any other pertinent migration guides as well.
 This section describes changes to contracts (classes, interfaces, methods, etc.) which are considered https://hibernate.org/community/compatibility-policy/#api[API].
 
 [[session-getLobHelper]]
-==== Session#getLobHelper
+=== Session#getLobHelper
 
 The `Session#getLobHelper` method has been marked as deprecated in favor of the static `Hibernate#getLobHelper` and will be removed in a future *major* version.
 
+[[ddl-changes]]
+== Changes to DDL generation
+
+This section describes changes to DDL generated by the schema export tooling.
+Such changes typically do not impact programs using a relational schema managed externally to Hibernate.
+
+[[single-table-check]]
+=== Automatic check constraints with single table inheritance mappings
+
+Previously, the non-nullability of the column mapped by an attribute declared `optional=false` by a subclass in a single table inheritance hierarchy was not enforced by the database.
+Hibernate now automatically generates DDL `check` constraints to enforce the non-nullability of such columns.
+