docs: cgroup-v1: use bullet lists for list of stat file tables

The stat file section contains three tables, where the leading texts for
them are subsection heading. Organize them in the bullet list, while
demoting headings into normal text.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>