Issue #2085957 by tankerjoe: Document that BookController methods return...

Issue #2085957 by tankerjoe: Document that BookController methods return render arrays, not strings.

core/modules/book/lib/Drupal/book/Controller/BookController.php

@@ -61,8 +61,8 @@ public static function create(ContainerInterface $container) {
   /**
    * Returns an administrative overview of all books.
    *
-   * @return string
-   *   A HTML-formatted string with the administrative page content.
+   * @return array
+   *   A render array representing the administrative page content.
    *
    */
   public function adminOverview() {
@@ -98,8 +98,8 @@ public function adminOverview() {
   /**
    * Prints a listing of all books.
    *
-   * @return string
-   *   A HTML-formatted string with the listing of all books content.
+   * @return array
+   *   A render array representing the listing of all books content.
    */
   public function bookRender() {
     $book_list = array();