Add help text

Closes #3426888

modules/custom_elements_ui/src/Form/EntityCustomElementsDisplayEditForm.php

@@ -88,6 +88,11 @@ class EntityCustomElementsDisplayEditForm extends CustomElementsEntityDisplayFor
         'view_mode_name' => $display->getMode(),
       ];
 
+      $form['help'] = [
+        '#type' => 'markup',
+        '#markup' => $this->t('<p><em>Layout builder</em> is enabled for this view mode. Thus, for this view mode the configured layout sections and the contained blocks are getting rendered as custom element. Configure the layout to customize how the element should be built.</p>'),
+      ];
+
       $form['manage_layout'] = [
         '#type' => 'link',
         '#title' => $this->t('Manage layout'),