ArduPlane: add comments on tailsitter pitch limits

ArduPlane/ArduPlane.cpp

@@ -774,9 +774,13 @@ void Plane::update_flight_mode(void)
         nav_roll_cd  = (channel_roll->get_control_in() / 4500.0) * roll_limit;
         nav_roll_cd = constrain_int32(nav_roll_cd, -roll_limit, roll_limit);
         float pitch_input = channel_pitch->norm_input();
+        // Scale from normalized input [-1,1] to centidegrees
         if (quadplane.is_tailsitter()) {
+            // For tailsitters, the pitch range is symmetrical: [-Q_ANGLE_MAX,Q_ANGLE_MAX]
             nav_pitch_cd = pitch_input * quadplane.aparm.angle_max;
         } else {
+            // For non-tailsitters, pitch is further constrained by LIM_PITCH_MIN/MAX which may impose
+            // tighter (and asymmetrical) limits than Q_ANGLE_MAX
             if (pitch_input > 0) {
                 nav_pitch_cd = pitch_input * MIN(aparm.pitch_limit_max_cd, quadplane.aparm.angle_max);
             } else {