HID: lg4ff - Document sysfs interface for range setting

Adds documentation of the sysfs interface used to set the wheel range.

Signed-off-by: Michal Malý <madcatxster@gmail.com>
Signed-off-by: Simon Wood <simon@mungewell.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>