Merge pull request #5717 from atwixfirster/schema-on-update-for-constraint

magento/devdocs#: schema.db does not support ON UPDATE action for constraints

Author: dobooth

guides/v2.3/extension-dev-guide/declarative-schema/db-schema.md

@@ -192,6 +192,9 @@ Attribute | Description
 `referenceColumn`| A column in the `referenceTable`
 `onDelete` | Foreign key trigger. The value must be `CASCADE`, `SET NULL`, or `NO ACTION`
 
+{: .bs-callout-info }
+To keep entity identifiers as immutable values, the declarative schema does not support `ON UPDATE` action for `constraint`.
+
 Example:
 
 ```xml
@@ -437,7 +440,7 @@ It is possible to drop a foreign key only if it exists in the `db_schema_whiteli
 In this example, Module A defines a new table with primary key `id_column`. Module B declares its own schema, in which it creates a new column (`new_id_column`) and changes the primary index to this column.
 Module B disables the original primary key and sets a new primary key with a `referenceId` value that is different from PRIMARY. Although this value is different, the real name of the primary key in the database remains PRIMARY.
 
- **Module A declaration**
+**Module A declaration**
 
 ```xml
 <schema xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
@@ -451,7 +454,7 @@ Module B disables the original primary key and sets a new primary key with a `re
 </schema>
 ```
 
- **Module B declaration**
+**Module B declaration**
 
 ```xml
 <schema xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"