pwm: tiecap: Document behaviour of hardware disable

pwm: tiecap: Document behaviour of hardware disable

According to David Lechner[1] disabling a tiecap PWM makes the PWM pin
an input. The reported problem is fixed in commit deaeeda2051f
("backlight: pwm_bl: Don't rely on a disabled PWM emiting inactive
state"). Document the behaviour in the driver for future reference.

[1] https://lore.kernel.org/linux-pwm/39a472c0-ba24-de7b-8783-a16a71b172cd@lechnology.com

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Reviewed-by: David Lechner <david@lechnology.com>
Link: https://lore.kernel.org/r/20250730080219.183181-2-u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>