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>

Author: Waiman-Long

Documentation/admin-guide/cgroup-v2.rst

@@ -364,6 +364,13 @@ constraint, a threaded controller must be able to handle competition
 between threads in a non-leaf cgroup and its child cgroups.  Each
 threaded controller defines how such competitions are handled.
 
+Currently, the following controllers are threaded and can be enabled
+in a threaded cgroup::
+
+- cpu
+- cpuset
+- perf_event
+- pids
 
 [Un]populated Notification
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