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.se


Signed-off-by: Hans de Goede <hdegoede@redhat.com>