diff --git a/core/modules/book/src/BookManagerInterface.php b/core/modules/book/src/BookManagerInterface.php
index 21008fd244f1ff86a8246acc60109c034c6e8601..1f4f131134c9d1243c180f55116a82d931fea4e2 100644
--- a/core/modules/book/src/BookManagerInterface.php
+++ b/core/modules/book/src/BookManagerInterface.php
@@ -180,15 +180,23 @@ public function getAllBooks();
   public function updateOutline(NodeInterface $node);
 
   /**
-   * Saves a single book entry.
+   * Saves a link for a single book entry to the book.
    *
    * @param array $link
-   *   The link data to save.
+   *   The link data to save. $link['nid'] must be set. Other keys in this array
+   *   get default values from
+   *   \Drupal\book\BookManagerInterface::getLinkDefaults(). The array keys
+   *   available to be set are documented in
+   *   \Drupal\book\BookOutlineStorageInterface::loadMultiple().
    * @param bool $new
-   *   Is this a new book.
+   *   Whether this is a link to a new book entry.
    *
    * @return array
-   *   The book data of that node.
+   *   The book entry link information. This is $link with values added or
+   *   updated.
+   *
+   * @see \Drupal\book\BookManagerInterface::getLinkDefaults()
+   * @see \Drupal\book\BookOutlineStorageInterface::loadMultiple()
    */
   public function saveBookLink(array $link, $new);