iio:Documentation: Add documentation for label channel attribute

If a label is defined for this channel add that to the channel specific
attributes. This is useful for userspace to be able to identify an
individual channel.

Signed-off-by: Cristian Pop <cristian.pop@analog.com>
Link: https://lore.kernel.org/r/20200928090959.88842-2-cristian.pop@analog.com


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