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/152Reported-by: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Documentation/ABI/testing/sysfs-class-power

@@ -480,6 +480,19 @@ Description:
 
 		Valid values: Represented in microvolts
 
+What:		/sys/class/power_supply/<supply_name>/cycle_count
+Date:		January 2010
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Reports the number of full charge + discharge cycles the
+		battery has undergone.
+
+		Access: Read
+
+		Valid values:
+			Integer > 0: representing full cycles
+			Integer = 0: cycle_count info is not available
+
 **USB Properties**
 
 What:		/sys/class/power_supply/<supply_name>/input_current_limit