Documentation - Explain variable Tx Power of PRC-77

[mrschick]

When merged this pull request will:

• Document the AN/PRC-77's variable transmit power depending on the selected frequency.
  

  acre2/addons/sys_prc77/radio/fnc_setCurrentChannel.sqf

  Lines 45 to 55 in 14c929f

  	private _power = 4000;
  	if (_frequency < 34 || _frequency > 50) then {
  	_power = 3500;
  	if (_frequency > 53) then {
  	_power = 3000;
  	};
  	if (_frequency > 71) then {
  	_power = 2600;
  	};
  	};