[media] DocBook/media: Improve G_EDID specification

When using VIDIOC_G_EDID there is a special case where start_blocks and
blocks are both set to 0. In that case the driver just has to set blocks to
the total number of available blocks and return 0.

Even though the drivers do this right and v4l2-compliance tests for it, it
turned out not to be documented in the spec. Fix this.

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