[media] DocBook media: update VIDIOC_CREATE_BUFS documentation

During the Seoul media workshop we decided to relax the VIDIOC_CREATE_BUFS
specification so it would no longer require drivers to validate the format
field since almost no driver did that anyway.

Instead drivers use the buffer size(s) based on the format type and the
corresponding format fields and will ignore any other fields. If the size
cannot be used an error is returned, otherwise the size is used as-is.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>