arg() doc states incorrect return value in Drupal 5

http://api.drupal.org/api/function/arg/5 It is stated here that FALSE is returned on failure, but in fact, nothing is returned. A test on arg($i) === FALSE, where $i is greater than the number of arguments, returns false, whereas arg($i) === NULL is true. Change docs to reflect behavior. (I believe the same issue exists for 4.7. Have not checked for other versions.)