[PATCH] docs: backlight: Clarify `actual_brightness`

[Mario Limonciello <superm1@kernel.org>]

From: Mario Limonciello <mario.limonciello@amd.com>

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
6ca017658b1f9 ("[PATCH] backlight: Backlight Class Improvements").

Cc: Lennart Poettering <lennart@poettering.net>
Cc: richard.purdie@linuxfoundation.org
Link: https://github.com/systemd/systemd/pull/36881
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
---
 Documentation/ABI/stable/sysfs-class-backlight | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Documentation/ABI/stable/sysfs-class-backlight b/Documentation/ABI/stable/sysfs-class-backlight
index 6102d6bebdf9a..909cacde85041 100644
--- a/Documentation/ABI/stable/sysfs-class-backlight
+++ b/Documentation/ABI/stable/sysfs-class-backlight
@@ -26,7 +26,9 @@ Date:		March 2006
 KernelVersion:	2.6.17
 Contact:	Richard Purdie <rpurdie@rpsys.net>
 Description:
-		Show the actual brightness by querying the hardware.
+		Show the actual brightness by querying the hardware. This
+		takes into account other factors besides the value programmed
+		for 'brightness' and may not match the value in 'brightness'.
 Users:		HAL
 
 What:		/sys/class/backlight/<backlight>/max_brightness
-- 
2.43.0