Plane: Use stall speed for roll stall prevention

The new AIRSPEED_STALL parameter wasn't being used for the roll stall
prevention, and no adjustment for bank angle was being made. This led to
a slight under-limiting of bank angle.

Fixed both issues by using the stall airspeed from the TECS, to avoid
repeating the same logic, and adjusted the description of the
AIRSPEED_STALL parameter accordingly. This improves consistency as it
uses AIRSPEED_STALL for all stall prevention calculations where it
applies, and sets up AIRSPEED_STALL to become a key parameter in further
stall prevention improvements.

Author: rubenp02

ArduPlane/Attitude.cpp

@@ -659,7 +659,8 @@ void Plane::update_load_factor(void)
     }
 #endif
 
-    float max_load_factor = sq(smoothed_airspeed / MAX(aparm.airspeed_min, 1));
+    float max_load_factor =
+        sq(smoothed_airspeed / MAX(TECS_controller.get_stall_airspeed(), 1));
 
     if (max_load_factor <= 1) {
         // our airspeed is below the minimum airspeed. Limit roll to
@@ -671,7 +672,7 @@ void Plane::update_load_factor(void)
         // load limit. Limit our roll to a bank angle that will keep
         // the load within what the airframe can handle. We always
         // allow at least 25 degrees of roll however, to ensure the
-        // aircraft can be manoeuvred with a bad airspeed estimate. At
+        // aircraft can be manoeuvered with a bad airspeed estimate. At
         // 25 degrees the load factor is 1.1 (10%)
         int32_t roll_limit = degrees(acosf(1.0f / max_load_factor))*100;
         if (roll_limit < 2500) {

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. Value is as an indicated (calibrated/apparent) airspeed.
+    // @Description: If stall prevention is enabled this speed is used for its calculations. If set to 0 then AIRSPEED_MIN is used instead. Typically set slightly higher than the true stall speed. Value is as an indicated (calibrated/apparent) airspeed.
     // @Units: m/s
     // @Range: 5 75
     // @User: Standard