hook_user() function signature fourth argument default value

http://drupaldocs.org/api/4.6/function/hook_user shows the function signature as:
hook_user($op, &$edit, &$user, $category)
but it can be called with only three parameters in some cases. It would be better to show the signature as:
hook_user($op, &$edit, &$user, $category='') or
hook_user($op, &$edit, &$user, $category=False)