Added description of "allow_no_delete" in migrations (#2853)

Author: mnocon

code_samples/data_migration/examples/delete_content.yaml

@@ -0,0 +1,8 @@
+-
+    type: content
+    mode: delete
+    match:
+        field: content_remote_id
+        value: __REMOTE_ID__
+        only_visible_content: true # Optional, default: true. If set to true, only visible content will be deleted. 
+    allow_no_delete: false # Optional, default: false. If set to true, the migration will not fail if no content is found using the specified criteria.

docs/content_management/data_migration/importing_data.md

@@ -242,6 +242,12 @@ In this case you assign the `parent_folder_location_id` reference name to the lo
 [[= include_file('code_samples/data_migration/examples/create_parent_and_child_content.yaml') =]]
 ```
 
+Use the `delete` mode to delete content items:
+
+``` yaml
+[[= include_file('code_samples/data_migration/examples/delete_content.yaml') =]]
+```
+
 ### Images
 
 The following example shows how to migrate an `example-image.png` located in `public/var/site/storage/images/3/8/3/0/383-1-eng-GB` without manually placing it in the appropriate path.