Perform alterations on Field API formatter types.
$info: An array of information on formatter types exposed by hook_field_formatter_info() implementations.
function hook_field_formatter_info_alter(&$info) {
// Add a setting to a formatter type.
$info['text_default']['settings'] += array(
'mymodule_additional_setting' => 'default value',
);
// Let a new field type re-use an existing formatter.
$info['text_default']['field types'][] = 'my_field_type';
}