iio: iio.h: use nested struct support on kernel-doc markup

Solve those Sphinx warnings:

    ./include/linux/iio/iio.h:270: warning: Function parameter or member 'scan_type.sign' not described in 'iio_chan_spec'
    ./include/linux/iio/iio.h:270: warning: Function parameter or member 'scan_type.realbits' not described in 'iio_chan_spec'
    ./include/linux/iio/iio.h:270: warning: Function parameter or member 'scan_type.storagebits' not described in 'iio_chan_spec'
    ./include/linux/iio/iio.h:270: warning: Function parameter or member 'scan_type.shift' not described in 'iio_chan_spec'
    ./include/linux/iio/iio.h:270: warning: Function parameter or member 'scan_type.repeat' not described in 'iio_chan_spec'
    ./include/linux/iio/iio.h:270: warning: Function parameter or member 'scan_type.endianness' not described in 'iio_chan_spec'

    ./include/linux/iio/iio.h:191: WARNING: Unexpected indentation.
    ./include/linux/iio/iio.h:192: WARNING: Block quote ends without a blank line; unexpected unindent.
    ./include/linux/iio/iio.h:198: WARNING: Definition list ends without a blank line; unexpected unindent.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

include/linux/iio/iio.h

@@ -183,18 +183,18 @@ struct iio_event_spec {
  * @address:		Driver specific identifier.
  * @scan_index:		Monotonic index to give ordering in scans when read
  *			from a buffer.
- * @scan_type:		sign:		's' or 'u' to specify signed or unsigned
- *			realbits:	Number of valid bits of data
- *			storagebits:	Realbits + padding
- *			shift:		Shift right by this before masking out
- *					realbits.
- *			repeat:		Number of times real/storage bits
- *					repeats. When the repeat element is
- *					more than 1, then the type element in
- *					sysfs will show a repeat value.
- *					Otherwise, the number of repetitions is
- *					omitted.
- *			endianness:	little or big endian
+ * @scan_type:		struct describing the scan type
+ * @scan_type.sign:		's' or 'u' to specify signed or unsigned
+ * @scan_type.realbits:		Number of valid bits of data
+ * @scan_type.storagebits:	Realbits + padding
+ * @scan_type.shift:		Shift right by this before masking out
+ *				realbits.
+ * @scan_type.repeat:		Number of times real/storage bits repeats.
+ *				When the repeat element is more than 1, then
+ *				the type element in sysfs will show a repeat
+ *				value. Otherwise, the number of repetitions
+ *				is omitted.
+ * @scan_type.endianness:	little or big endian
  * @info_mask_separate: What information is to be exported that is specific to
  *			this channel.
  * @info_mask_separate_available: What availability information is to be