Implements hook_field_instance_settings_form().
function link_field_instance_settings_form($field, $instance) {
$form['title'] = array(
'#type' => 'radios',
'#title' => t('Allow link title'),
'#default_value' => isset($instance['settings']['title']) ? $instance['settings']['title'] : DRUPAL_OPTIONAL,
'#options' => array(
DRUPAL_DISABLED => t('Disabled'),
DRUPAL_OPTIONAL => t('Optional'),
DRUPAL_REQUIRED => t('Required'),
),
);
return $form;
}