[media] DocBook media: v4l2_sdr_format buffersize field

New field buffersize was added to inform application maximum
buffer size used. Add it to documentation too.

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>