Document that BookController methods return render arrays, not strings

Two BookController methods claim to return a string, when in fact they return a render array.

  /**
   * Returns an administrative overview of all books.
   *
   * @return string
   *   A HTML-formatted string with the administrative page content.
   *
   */
  public function adminOverview() {
...
    return array(

and

  /**
   * Prints a listing of all books.
   *
   * @return string
   *   A HTML-formatted string with the listing of all books content.
   */
  public function bookRender() {
...
    return array(