feat(copy): adds documentation for datasource on_copy

Author: Niels Verbeek

forms/data-sources.md

@@ -37,6 +37,13 @@ For the label, it's possible to use a dict with translated values.
 
 Note: If the labels returned from`get_data()` depend on the current user's language, you need to return a `dict` with the language code as keys instead of translating the value yourself. Returning already translated values is not supported, as it would break caching and validation.
 
+### `on_copy`-method
+
+Must return tuple with a slug and a label. This method is called when a form answer with dynamic options is copied. It is used to optionally alter the slug and label of the dynamic option.
+By default it will return the input slug and label without modifying it.
+
+When returning a None value for the slug the dynamic option will be discarded.
+
 #### Arguments
 
 * `user`: The OIDC user object for the request