[media] vidioc-g-sliced-vbi-cap.rst: make tables fit on LaTeX output

The tables don't fit at the page on this file. As noticed
before, Sphinx (or LaTeX?) does a crap job on tables with
cell span, and some work has to be done to make it fit.

Move the see also reference to a footnote, break one paragraph
into two and adjust the table columns to make it visible.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>