If you enter "rcntp" it shows the list of the possible parameters:

 | vm06:~ # rcntp
 | Usage: /usr/sbin/rcntp {start|stop|status|try-restart|restart|restart-iburst
 |                              |force-reload|reload|probe|ntptimeset}

Though, the parameter "restart-iburst" is not accepted:

 | vm06:~ # rcntp restart-iburst
 | Usage: /usr/sbin/rcntp {start|stop|status|try-restart|restart|restart-iburst
 |                              |force-reload|reload|probe|ntptimeset}

Looking at the script "/usr/sbin/rcntp" gives that it accepts the parameter
"try-restart-iburst" but not "restart-iburst". And indeed, the option
"try-restart-iburst" works well.

So it seems that the script only displays the wrong parameter in the help line.