Document higher bound of pwm frequency

https://github.com/harp-tech/device.behavior/blob/ddf71cfbec248f1cd7b7abaa9c477ac58e4a4dae/device.yml#L131

I was playing with the pwm frequency and it seems to cap at 10khz after which the board returns a write error. @artursilva0 @filcarv Can you confirm this behavior?