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: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>