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

Cc: Lee Jones <[e-mail]>
Cc: Lennart Poettering <[e-mail]>
Cc: [e-mail]
Link: https://github.com/systemd/systemd/pull/36881
Reviewed-by: Harry Wentland <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mario Limonciello <[e-mail]>

Documentation/ABI/stable/sysfs-class-backlight

···
 KernelVersion:	2.6.17
 Contact:	Richard Purdie <[e-mail]>
 Description:
-		Show the actual brightness by querying the hardware.
+		Show the actual brightness by querying the hardware. Due
+		to implementation differences in hardware this may not
+		match the value in 'brightness'. For example some hardware
+		may treat blanking differently or have custom power saving
+		features. Userspace should generally use the values in
+		'brightness' to make decisions.
 Users:		HAL
 
 What:		/sys/class/backlight/<backlight>/max_brightness