media: buffer.rst: fix a PDF output issue

Sphinx/LaTeX doesn't work well with literal blocks.
Due to that, V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF ends
being writing outside the table (and even outside the page).

We need to not only change the table size and font size, but
also to change the message, in order to avoid this bug.

So, improve the text a little bit, while ensuring that the
literal will be written at the beginning of the second line.

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