Update PX4 docs with information about ability to switch off airspeed scaling

[roman-dvorak]

Airspeed scaling is possible to turn off with parameter. In px4 docs it is not mentioned.

https://dev.px4.io/master/en/flight_stack/controller_diagrams.html#airspeed-scaling

[kaklik]

I want to fix that, but I do not know the exact implementation. e.g. if setting the FW_ARSP_SCALE_EN to zero disable to scale everything (rudder, elevator, ailerons) or it disables the airspeed scaling of the controls related to the rotor (elevator, ailerons) only. Fundamentally it would be logical to leave the rudder control of airspeed scaling for rudder in use even in the case of the autogyro.