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>