Enables a pager for the query.
$limit: An integer specifying the number of elements per page. If passed a false value (FALSE, 0, NULL), the pager is disabled.
$element: An optional integer to distinguish between multiple pagers on one page. If not provided, one is automatically calculated.
\Drupal\Core\Entity\Query\QueryInterface The called object.
public function pager($limit = 10, $element = NULL);