media: Documentation: media: Refer to mbus format documentation from CSI-2 docs

The media bus formats to be used on serial busses are documented but there
was no reference from CSI-2 documentation. Add that now.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>