pwm: Better document return value of pwm_round_waveform_might_sleep()

pwm: Better document return value of pwm_round_waveform_might_sleep()

Better explain how pwm_round_waveform_might_sleep() (and so the
respective lowlevel driver callback) is supposed to round and the
meaning of the return value.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/db84abf1e82e4498fc0e7c318d2673771d0039fe.1744120697.git.ukleinek@kernel.org
[ukleinek: Fix a rst formatting issue reported by Stephen Rothwell]
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>