Issue #3087486 by AkashkumarOSL, kim.pepper, catch: Update docs in PagerManagerInterface

   * that invokes an external datastore with an SQL-like syntax:

   * @code

   *   // First find the total number of items and initialize the pager.

   *   $where = "status = 1";

   *   $total = mymodule_select("SELECT COUNT(*) FROM data " . $where)->result();

   *   $total = mymodule_select("SELECT COUNT(*) FROM data WHERE status = 1")->result();

   *   $num_per_page = \Drupal::config('mymodule.settings')->get('num_per_page');

   *   $pager = \Drupal::service('pager.manager')->createPager($total, $num_per_page);

   *   $page = $pager->getCurrentPage();