diff --git a/core/modules/book/book.js b/core/modules/book/book.js
index e67c6a2..249f5a6 100644
--- a/core/modules/book/book.js
+++ b/core/modules/book/book.js
@@ -8,7 +8,12 @@
   "use strict";
 
   /**
+   * Adds summaries to the book outline form.
+   *
    * @type {Drupal~behavior}
+   *
+   * @prop {Drupal~behaviorAttach} attach
+   *   Attaches summary behavior to book outline forms.
    */
   Drupal.behaviors.bookDetailsSummaries = {
     attach: function (context) {