cgroup: add warning about RT not being supported on cgroup2

We haven't yet figured out what to do with RT threads on cgroup2.
Document the limitation.

v2: Included the warning about system management software behavior as
    suggested by Michael.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>