iio: imu: adis: add a note better explaining state_lock

The 'state_lock' mutex was renamed from 'txrx_lock' in a previous patch and
is intended to be used by ADIS drivers to protect the state of devices
during consecutive R/W ops.
The initial patch that introduced this change did not do a good [well, any]
job at explaining this. This patch adds a comment to the 'state_lock'
better explaining it's use.
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

include/linux/iio/imu/adis.h

@@ -98,6 +98,17 @@ struct adis {
 	const struct adis_data	*data;
 	struct adis_burst	*burst;
 
+	/**
+	 * The state_lock is meant to be used during operations that require
+	 * a sequence of SPI R/W in order to protect the SPI transfer
+	 * information (fields 'xfer', 'msg' & 'current_page') between
+	 * potential concurrent accesses.
+	 * This lock is used by all "adis_{functions}" that have to read/write
+	 * registers. These functions also have unlocked variants
+	 * (see "__adis_{functions}"), which don't hold this lock.
+	 * This allows users of the ADIS library to group SPI R/W into
+	 * the drivers, but they also must manage this lock themselves.
+	 */
 	struct mutex		state_lock;
 	struct spi_message	msg;
 	struct spi_transfer	*xfer;