A PASSENGER TRANSPORT EXPERIENCED SEVERE TURBULENCE DURING AN EN ROUTE DESCENT TO DESTINATION. THE FLIGHT'S FORECAST INDICATED THUNDERSTORM DEVELOPMENT IN THE DESTINATION VICINITY. THE CREW HEARD DESTINATION ATIS ANNOUNCE SEVERE TURBULENCE ON APPROACH BUT RECALLED NO ATC ADVISORY STATING TURBULENCE WAS SEVERE. THE PILOT COMPLIED WITH ATC'S REQUEST TO SLOW FOR TRAFFIC SEPARATION, PLACING THE SPEED BELOW THAT RECOMMENDED FOR TURBULENCE PENETRATION. THE AIRPLANE WAS TURNING TO AVOID CLOUDS AND VIRGA, WITH SPEEDBRAKES EXTENDED AND AUTOPILOT ENGAGED WHEN IT ENTERED INCREASING TURBULENCE. THE AIRPLANE PITCHED UP 20 DEGREES AND ROLLED TO 45 DEGREES. THE PILOT RIGHTED THE AIRPLANE AND STOPPED THE DESCENT 500 FEET BELOW ASSIGNED ALTITUDE. SPEED BRAKE EXTENSION INCREASES ROLL RATE. AUTOPILOT USE IN SEVERE TURBULENCE IS RECOMMENDED ONLY IN THE PITCH-HOLD MODE.