Add API documentation

I should add an on_the_web.api.php file that documents use of hook_on_the_web_get_services_alter().

Here's a good example:

/**
 * Implements hook_on_the_web_get_services_alter().
 * Puts facebook at the very front of the list of icons.
 */
function mymodule_on_the_web_get_services_alter(&$services) {
  // Pull out facebook.
  $face = $services['facebook'];
  // Remove facebook.
  unset($services['facebook']);
  // Put facebook at the begining.
  $services = array('facebook' => $face)+$services;
}