docs: ABI: sysfs-class-power: Documented cycle_count property

Commit c955fe8e ("POWER: Add support for cycle_count") added a
POWER_SUPPLY_PROP_CYCLE_COUNT "cycle_count" property to the set of
standard power-supply properties, but this was never documented,
document it now.

Link: https://gitlab.freedesktop.org/upower/upower/-/issues/152


Reported-by: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>