docs: fs: proc.rst: convert a new chapter to ReST

A new chapter was added to proc.rst. Adjust the markups
to avoid this warning:

	Documentation/filesystems/proc.rst:2194: WARNING: Inconsistent literal block quoting.

And to properly mark the code-blocks there.

Fixes: 37e7647a ("docs: proc: add documentation for "hidepid=4" and "subset=pid" options and new mount behavior")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/de67ec04a2e735f4450eb3ce966f7d80b9438244.1592895969.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>