Implements hook_field_formatter_settings_summary().
function text_field_formatter_settings_summary($field, $instance, $view_mode) {
$display = $instance['display'][$view_mode];
$settings = $display['settings'];
$summary = '';
if (strpos($display['type'], '_trimmed') !== FALSE) {
$summary = t('Trimmed limit: @trim_length characters', array(
'@trim_length' => $settings['trim_length'],
));
}
return $summary;
}