backlight(8): Add note that with option it print the current brightness.

MFC after: 3 days
PR: 253737
Approved by: re (gjb)

(cherry picked from commit 1df30489a8f7083c98010c94d9ce522f9e8213dc)
(cherry picked from commit f21c0366f532888bec164717a93964610ab6baf6)