RS485 documentation: add 16C950 UART description

2. HARDWARE-RELATED CONSIDERATIONS

   Some CPUs (e.g., Atmel AT91) contain a built-in half-duplex mode capable of

   automatically controlling line direction by toggling RTS. That can used to

   control external half-duplex hardware like an RS485 transceiver or any

   RS232-connected half-duplex device like some modems.

   Some CPUs/UARTs (e.g., Atmel AT91 or 16C950 UART) contain a built-in

   half-duplex mode capable of automatically controlling line direction by

   toggling RTS or DTR signals. That can be used to control external

   half-duplex hardware like an RS485 transceiver or any RS232-connected

   half-duplex devices like some modems.

   For these microcontrollers, the Linux driver should be made capable of

   working in both modes, and proper ioctls (see later) should be made