Give a brief overview of the cgroup.kill functionality.

Link: https://lore.kernel.org/r/20210503143922.3093755-2-brauner@kernel.org
Cc: Roman Gushchin <guro@fb.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: cgroups@vger.kernel.org
Reviewed-by: NShakeel Butt <shakeelb@google.com>
Signed-off-by: NChristian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: NTejun Heo <tj@kernel.org>