[media] dev-raw-vbi.rst: add a footnote for the count limits

[media] dev-raw-vbi.rst: add a footnote for the count limits

There's a bug with LaTeX output on flat-tables with Sphinx 1.4.5
that prevents references at a cell span to be broken. As the
text is indeed too long, it makes sense to place the reference
to the pictures showing the VBI limits as a footnote.

That makes the text easier to read and also solves the issue
with LaTeX output.

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