media: docs: sliced-vbi: fix V4L2_SLICED_WSS_625 docs

While fixing issues with PDF generation, I noticed that
bit 8 was missing for WSS 625 format.

While here, convert the literal block into a tables,
as it becomes more visible.

I opted to move the payload into a separate table, as
Sphinx has troubles with complex nested tables.

This way, it should work fine on both html and LaTeX/PDF
formats.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>