Add NAV RADIO Audio knob description

[frankkopp]

https://discord.com/channels/738864299392630914/748761999747579944/1302755249605443595

Chaoz [Z+1] — Today at 11:04 PM
deactivate this knob

on the RMP

tracernz [Z+13] — Today at 11:17 PM
Yeah, the volume knob in/out turns the NAV RADIO audio on/off, and rotating controls it's volume, the selector down there decides which NAV radio you're listening to, and the VOICE button toggles on/off filtering of the morse code (so you can hear ASOS broadcasts for instance)
All that is implemented