[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>

Documentation/media/uapi/v4l/dev-raw-vbi.rst

@@ -196,10 +196,9 @@ and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does
 	  driver. Anyway, drivers may not support single field capturing and
 	  return both count values non-zero.
 
-	  Both ``count`` values set to zero, or line numbers outside the
-	  bounds depicted in :ref:`vbi-525` and :ref:`vbi-625`, or a
-	  field image covering lines of two fields, are invalid and shall
-	  not be returned by the driver.
+	  Both ``count`` values set to zero, or line numbers are outside the
+	  bounds depicted\ [#f4]_, or a field image covering lines of two
+	  fields, are invalid and shall not be returned by the driver.
 
 	  To initialize the ``start`` and ``count`` fields, applications
 	  must first determine the current video standard selection. The
@@ -352,3 +351,6 @@ another process.
    Most VBI services transmit on both fields, but some have different
    semantics depending on the field number. These cannot be reliable
    decoded or encoded when ``V4L2_VBI_UNSYNC`` is set.
+
+.. [#f4]
+   The valid values ar shown at :ref:`vbi-525` and :ref:`vbi-625`.