diff --git a/core/includes/common.inc b/core/includes/common.inc
index ef2bf02..4bdc258 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -4189,6 +4189,9 @@ function drupal_render_collect_attached($elements, $return = FALSE) {
 /**
  * Collects cache tags for an element and its children into a single array.
  *
+ * The cache tags array is returned in a format that is valid for
+ * \Drupal\Core\Cache\CacheBackendInterface::set().
+ *
  * When caching elements, it is necessary to collect all cache tags into a
  * single array, from both the element itself and all child elements. This
  * allows items to be invalidated based on all tags attached to the content