docs: iio: mark "repeat" sysfs attribute as optional

Show that this field is optional, just like the shift value.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Link: https://lore.kernel.org/r/20210215143511.25471-6-luca@lucaceresoli.net


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>