ABI: testing: sysfs-class-power: Document absence of "present" property

Document how the absence of the "present" property in the sysfs
power_supply class should be handled.

Link: https://gitlab.freedesktop.org/upower/upower/-/merge_requests/173
Cc: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: NBastien Nocera <hadess@hadess.net>
Signed-off-by: NSebastian Reichel <sebastian.reichel@collabora.com>

Documentation/ABI/testing/sysfs-class-power

@@ -437,7 +437,8 @@ What:		/sys/class/power_supply/<supply_name>/present
 Date:		May 2007
 Contact:	linux-pm@vger.kernel.org
 Description:
-		Reports whether a battery is present or not in the system.
+		Reports whether a battery is present or not in the system. If the
+		property does not exist, the battery is considered to be present.
 
 		Access: Read