media: v4l: Documentation: Serial busses use parallel mbus codes

When the support for serial busses was introduced in V4L2, it was decided
to use the existing parallel bus media bus pixel codes to describe them.
While this was a practical choice at the time, it necessitates choosing
which one of the many parallel mbus pixel codes to use, for on the serial
busses these formats are effectively all equivalent.

The practice has always been to use the pixel code that describes a bus
that transfers a single sample per clock. Document this.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>