docs: backlight: Clarify `actual_brightness`

Currently userspace software systemd treats `brightness` and
`actual_brightness` identically due to a bug found in an out of tree
driver.

This however causes problems for in-tree drivers that use brightness
to report user requested `brightness` and `actual_brightness` to report
what the hardware actually has programmed.

Clarify the documentation to match the behavior described in commit
6ca01765 ("[PATCH] backlight: Backlight Class Improvements").

Cc: Lee Jones <lee@kernel.org>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: richard.purdie@linuxfoundation.org
Link: https://github.com/systemd/systemd/pull/36881


Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Link: https://lore.kernel.org/r/20250415192101.2033518-1-superm1@kernel.org


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>