pwm: Add kernel doc for members added to pwm_ops recently

pwm: Add kernel doc for members added to pwm_ops recently

The callbacks for lowlevel pwm drivers were expanded to handle the new
waveform abstraction. When doing that I missed to expand the kernel doc
description. This is catched up here.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/linux-next/20241001135207.125ca7af@canb.auug.org.au
Fixes: 17e40c25158f ("pwm: New abstraction for PWM waveforms")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/20241001085138.1025818-2-u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>