docs: ABI: charge_control_end_threshold may not support all values

Some laptops (for example Toshiba Satellite Z830) only supports some fixed
values. Allow for this and document the expected behaviour in such cases.
Acked-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Arvid Norlander <lkml@vorpal.se>
Link: https://lore.kernel.org/r/20220902180037.1728546-4-lkml@vorpal.seSigned-off-by: Hans de Goede <hdegoede@redhat.com>

Documentation/ABI/testing/sysfs-class-power

@@ -364,7 +364,10 @@ Date:		April 2019
 Contact:	linux-pm@vger.kernel.org
 Description:
 		Represents a battery percentage level, above which charging will
-		stop.
+		stop. Not all hardware is capable of setting this to an arbitrary
+		percentage. Drivers will round written values to the nearest
+		supported value. Reading back the value will show the actual
+		threshold set by the driver.
 
 		Access: Read, Write