Documentation: devres: add missing LED helpers

Documentation: devres: add missing LED helpers

Add some missing device-managed helpers of led to devres.rst.

devm_led_classdev_register_ext() is introduced by commit b2b998c0f944 ("leds:
class: Improve LED and LED flash class registration API").

devm_led_trigger_register() is introduced by commit 9534cc31dda2 ("leds: core:
add managed version of led_trigger_register").

devm_of_led_get() is introduced by commit e389240ad992 ("leds: Add managed
API to get a LED from a device driver").

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20221103021331.1007699-1-yangyingliang@huawei.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>