Bluetooth: Add ABI doc for sysfs reset

The functionality was implemented in commit 0f8a0013 ("Bluetooth:
Allow reset via sysfs")

Fixes: 0f8a0013 ("Bluetooth: Allow reset via sysfs")
Signed-off-by: Hsin-chen Chuang <chharry@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>