pwm: twl-led: Document some limitations and link to the reference manual

I found these just from reading the reference manual and the driver
source. It's unclear to me if there are glitches when updating the ON
and OFF registers.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>