pwm: Adapt sysfs API documentation to reality

Most low-level PWM drivers support duty_cycle == period, and so does the
sysfs API. Also polarity can be changed for enabled PWMs since commit
39100cee

 ("pwm: Switch to the atomic API").

Reported-by: Jens Gehrlein <J.Gehrlein@eckelmann.de>
Link: https://lore.kernel.org/r/20230911154454.675057-1-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>