doc: cgroup: improve formatting of mem stats

If there is an empty line between item and description
Sphinx does not emphasize the item. First half of the
list does not have the empty line and is emphasized
correctly.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lore.kernel.org/r/20200228000653.1572553-3-kuba@kernel.org


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