iio: core: Enhance the kernel doc of modes and currentmodes iio_dev entries

Let's provide more details about these two variables because their
understanding may not be straightforward for someone not used to the IIO
subsystem internal logic. The different modes will soon be also be more
documented for the same reason.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20220207143840.707510-2-miquel.raynal@bootlin.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

include/linux/iio/iio.h

@@ -488,8 +488,15 @@ struct iio_buffer_setup_ops {
 
 /**
  * struct iio_dev - industrial I/O device
- * @modes:		[DRIVER] operating modes supported by device
- * @currentmode:	[INTERN] current operating mode
+ * @modes:		[DRIVER] bitmask listing all the operating modes
+ *			supported by the IIO device. This list should be
+ *			initialized before registering the IIO device. It can
+ *			also be filed up by the IIO core, as a result of
+ *			enabling particular features in the driver
+ *			(see iio_triggered_event_setup()).
+ * @currentmode:	[INTERN] operating mode currently in use, may be
+ *			eventually checked by device drivers but should be
+ *			considered read-only as this is a core internal bit
  * @dev:		[DRIVER] device structure, should be assigned a parent
  *			and owner
  * @buffer:		[DRIVER] any buffer present