Issue #3425849 by jackwrfuller, joachim: DraggableListBuilderTrait needs to...

Issue #3425849 by jackwrfuller, joachim: DraggableListBuilderTrait needs to say that classes using it must implement FormInterface

core/lib/Drupal/Core/Entity/DraggableListBuilderTrait.php

@@ -7,8 +7,8 @@
 /**
  * Provides a trait for draggable listings of entities.
  *
- * Classes using this trait are expected to set the $formBuilder property in
- * their constructor.
+ * Classes using this trait must implement \Drupal\Core\Form\FormInterface and
+ * are expected to set the $formBuilder property in their constructor.
  */
 trait DraggableListBuilderTrait {