docs/cgroup: Add the list of threaded controllers to cgroup-v2.rst

The cgroup-v2 file mentions the concept of threaded controllers which can
be used in a threaded cgroup. However, it doesn't mention clearly which
controllers are threaded leading to some confusion about what controller
can be used requiring some experimentation. Clear this up by explicitly
listing the controllers that can be used currently in a threaded cgroup.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>