Depsgraph: Make a note about why is it OK to pass original base in relations builder

source/blender/depsgraph/intern/builder/deg_builder_relations_view_layer.cc

@@ -73,6 +73,10 @@ void DepsgraphRelationBuilder::build_view_layer(Scene *scene, ViewLayer *view_la
 	/* Setup currently building context. */
 	scene_ = scene;
 	/* Scene objects. */
+	/* NOTE: Nodes builder requires us to pass CoW base because it's being
+	 * passed to the evaluation functions. During relations builder we only
+	 * do NULL-pointer check of the base, so it's fine to pass original one.
+	 */
 	LINKLIST_FOREACH(Base *, base, &view_layer->object_bases) {
 		build_object(base, base->object);
 	}