Documentation problem with hook_watchdog

Sorry, I talked about this code :

<?php
function hook_watchdog($log_entry) {
  global $base_url, $language;

  $severity_list = array(
    WATCHDOG_EMERG    => t('Emergency'),
    WATCHDOG_ALERT    => t('Alert'),
    WATCHDOG_CRITICAL => t('Critical'),
    WATCHDOG_ERROR    => t('Error'),
    WATCHDOG_WARNING  => t('Warning'),
    WATCHDOG_NOTICE   => t('Notice'),
    WATCHDOG_INFO     => t('Info'),
    WATCHDOG_DEBUG    => t('Debug'),
  );

  $to = 'someone@example.com';
  $params = array();
  $params['subject'] = t('[@site_name] @severity_desc: Alert from your web site', array(
      '@site_name' => variable_get('site_name', 'Drupal'),
      '@severity_desc' => $severity_list[$log_entry['severity']]
  ));

  $params['message']  = "\nSite:         @base_url";
  $params['message'] .= "\nSeverity:     (@severity) @severity_desc";
  $params['message'] .= "\nTimestamp:    @timestamp";
  $params['message'] .= "\nType:         @type";
  $params['message'] .= "\nIP Address:   @ip";
  $params['message'] .= "\nRequest URI:  @request_uri";
  $params['message'] .= "\nReferrer URI: @referer_uri";
  $params['message'] .= "\nUser:         (@uid) @name";
  $params['message'] .= "\nLink:         @link";
  $params['message'] .= "\nMessage:      \n\n@message";

  $params['message'] = t($params['message'], array(
    '@base_url'      => $base_url,
    '@severity'      => $log_entry['severity'],
    '@severity_desc' => $severity_list[$log_entry['severity']],
    '@timestamp'     => format_date($log_entry['timestamp']),
    '@type'          => $log_entry['type'],
    '@ip'            => $log_entry['ip'],
    '@request_uri'   => $log_entry['request_uri'],
    '@referer_uri'   => $log_entry['referer'],
    '@uid'           => $log_entry['user']->uid,
    '@name'          => $log_entry['user']->name,
    '@link'          => strip_tags($log_entry['link']),
    '@message'       => strip_tags($log_entry['message']),
  ));

  drupal_mail('emaillog', 'log', $to, $language, $params);
}
?>