doc/admin-guide/cgroup-v2: use tables

These two places are rendered like a table in the source (rst) code,
but they are seen as plain text by formatters, and thus are joined
together into a single line, e.g.:

> “root” - a partition root “member” - a non-root member of a partition

This is definitely not what was intended.

To fix, use table formatting, like in other places.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20210120001824.385168-9-kolyshkin@gmail.com


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