docs: iio: document the 'index' attribute too

Two out of three attributes are documented, document the third one too.
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Link: https://lore.kernel.org/r/20210215143511.25471-4-luca@lucaceresoli.netSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/driver-api/iio/buffers.rst

@@ -34,6 +34,7 @@ attributes of the following form:
 * :file:`enable`, used for enabling a channel. If and only if its attribute
   is non *zero*, then a triggered capture will contain data samples for this
   channel.
+* :file:`index`, the scan_index of the channel.
 * :file:`type`, description of the scan element data storage within the buffer
   and hence the form in which it is read from user space.
   Format is [be|le]:[s|u]bits/storagebitsXrepeat[>>shift] .