media: v4l2-fwnode.h: better describe bus union at fwnode endpoint struct

Better document the bus union at struct v4l2_fwnode_endpoint.
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

include/media/v4l2-fwnode.h

@@ -81,7 +81,17 @@ struct v4l2_fwnode_bus_mipi_csi1 {
  * struct v4l2_fwnode_endpoint - the endpoint data structure
  * @base: fwnode endpoint of the v4l2_fwnode
  * @bus_type: bus type
- * @bus: bus configuration data structure
+ * @bus: union with bus configuration data structure
+ * @bus.parallel: embedded &struct v4l2_fwnode_bus_parallel.
+ *		  Used if the bus is parallel.
+ * @bus.mipi_csi1: embedded &struct v4l2_fwnode_bus_mipi_csi1.
+ *		   Used if the bus is MIPI Alliance's Camera Serial
+ *		   Interface version 1 (MIPI CSI1) or Standard
+ *		   Mobile Imaging Architecture's Compact Camera Port 2
+ *		   (SMIA CCP2).
+ * @bus.mipi_csi2: embedded &struct v4l2_fwnode_bus_mipi_csi2.
+ *		   Used if the bus is MIPI Alliance's Camera Serial
+ *		   Interface version 2 (MIPI CSI2).
  * @link_frequencies: array of supported link frequencies
  * @nr_of_link_frequencies: number of elements in link_frequenccies array
  */