serial: 8250-fsl: Expand description of the MPC83xx UART's misbehaviour

After working quite a bit on erratic behaviour of the MPC83xx UART I
(think I) understood the problem. Expand the description accoringly to
conserve the knowledge for the future.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20230524122754.481816-2-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>