Documentation: leds: Add "rgb:status" path

The path /sys/class/leds/rgb:status is already widely used with the
qcom-lpg driver and others. Document it.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Link: https://lore.kernel.org/r/20230414-pmi632-v3-3-079d2cada699@z3ntu.xyz


Signed-off-by: Lee Jones <lee@kernel.org>