[media] DocBook media: fix broken FIELD_ALTERNATE description

The sizeimage is that of a single field, not that of a full frame.
That makes no sense, and in fact all drivers supporting ALTERNATE will
set sizeimage to that of a field.

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