pwm: Reword docs about pwm_apply_state()

The main issue is that the current documentation talks about the
non-existent function pwm_get_last_applied_state. (This was right in the
context of
https://lore.kernel.org/linux-pwm/20210406073036.26857-1-u.kleine-koenig@pengutronix.de/


but was then missed to adapt when this patch was reduced to a
documentation update.)

While at is also clarify "last applied PWM state" to "PWM state that was
passed to the last invocation of pwm_apply_state()" to better
distinguish to the last actually implemented state and reword to drop a
word repetition.

Fixes: 1a7a6e80 ("pwm: Clarify which state pwm_get_state() returns")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>