[uart] Document that TX and RX should be disabled when configuring loopbacks

### Description

I ran into a lot of trouble when testing the UART loopbacks because phantom bytes would appear if loopbacks were configured when TX and RX were still enabled. The problem was something to do with the TX and RX lines not being pinmuxed and therefore not being pulled high (idle). When the TX and RX were connected, this low signal became a `START` bit and an `0xff` byte appeared.

It's a user error to expect the UART to work before the lines are pinmuxed, but the loopback shouldn't need those lines so it was unexpected to me. We should document this to prevent others making the same mistake.

Opening this issue so I don't forget again.