diff --git a/core/includes/common.inc b/core/includes/common.inc
index c9fdd6f..99b8962 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -1098,6 +1098,9 @@ function drupal_render_children(&$element, $children_keys = NULL) {
       $output .= drupal_render($element[$key]);
     }
   }
+  // $output is the concatenation of multiple render calls that are not
+  // guaranteed to be safe, but that have been generated through
+  // \Drupal\Core\Render\RendererInterface::render() and must be printed as-is.
   return SafeMarkup::set($output);
 }