add CascadeDelete flag and explain effect on clone process

guides/plugins/plugins/framework/data-handling/add-complex-data-to-existing-entities.md

@@ -88,6 +88,7 @@ Let's start with the `CustomExtension` class by adding a new field in the `exten
 namespace Swag\BasicExample\Extension\Content\Product;
 use Shopware\Core\Content\Product\ProductDefinition;
 use Shopware\Core\Framework\DataAbstractionLayer\EntityExtension;
+use Shopware\Core\Framework\DataAbstractionLayer\Field\Flag\CascadeDelete;
 use Shopware\Core\Framework\DataAbstractionLayer\Field\OneToOneAssociationField;
 use Shopware\Core\Framework\DataAbstractionLayer\FieldCollection;
 
@@ -96,7 +97,7 @@ class CustomExtension extends EntityExtension
     public function extendFields(FieldCollection $collection): void
     {
         $collection->add(
-            new OneToOneAssociationField('exampleExtension', 'id', 'product_id', ExampleExtensionDefinition::class, true)
+            (new OneToOneAssociationField('exampleExtension', 'id', 'product_id', ExampleExtensionDefinition::class, true))->addFlags(new CascadeDelete())
         );
     }
 
@@ -115,6 +116,9 @@ As you can see, we're adding a new `OneToOneAssociationField`. Its parameters ar
 * `referenceClass`: The class name of the definition that we want to connect via the association.
 * `autoload`: As the name suggests, this parameter defines if this association should always be loaded by default when the product is loaded. In this case, we definitely want that.
 
+Associations marked with the `CascadeDelete` flag are considered in the clone process. Take a look at the [clone behaviour](../../../../../resources/references/adr/2020-07-02-control-clone-behavior.md)
+If the associated entity should not be cloned when cloning a product having it, change it to `CascadeDelete(false)` or remove the flag.
+
 #### Creating ExampleExtensionDefinition
 
 You most likely noticed the new class `ExampleExtensionDefinition`, which we're going to create now. It will contain the actual string field that we wanted to add to the product.