What does "short brake" mean?
It is "short-circuit" -- the Control Function table on page 4 says that you can make it actively drive both outputs to the same level by either driving both inputs HIGH, or driving one input HIGH and the other LOW while commanding the PWM low.  This shorts the motor out via the power or ground rail.

You should be able to do this continuously -- the main limit is the temperature of the motor windings and FETs as that's what dissipates the brake energy if something is trying to spin the rotor at the time.