Plane: Update AIRSPEED_STALL parameter description

Updated the AIRSPEED_STALL parameter description to explain its effects
on the landing final

ArduPlane/Parameters.cpp

@@ -323,7 +323,7 @@ const AP_Param::Info Plane::var_info[] = {
 
     // @Param: AIRSPEED_STALL
     // @DisplayName: Stall airspeed
-    // @Description: If stall prevention is enabled this speed is used to calculate the minimum airspeed while banking. If this is set to 0 then the stall speed is assumed to be the minimum airspeed speed. Typically set slightly higher then true stall speed.
+    // @Description: If stall prevention is enabled this speed is used to calculate the minimum airspeed while banking. It is also used during landing final as the minimum airspeed that can be demanded by the TECS, which allows using TECS_LAND_ARSPD or LAND_PF_ARSPD to achieve landings slower than AIRSPEED_MIN. If this is set to 0 then the stall speed is assumed to be the minimum airspeed speed. Typically set slightly higher then true stall speed.
     // @Units: m/s
     // @Range: 5 75
     // @User: Standard