docs/cgroup: add entry for cgroup.kill

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: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>