docs/cgroup: add entry for cgroup.kill

Give a brief overview of the cgroup.kill functionality.

Link: https://lore.kernel.org/r/[e-mail]
Cc: Roman Gushchin <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Cc: [e-mail]
Reviewed-by: Shakeel Butt <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

Documentation/admin-guide/cgroup-v2.rst

···
 	it's possible to delete a frozen (and empty) cgroup, as well as
 	create new sub-cgroups.
 
+  cgroup.kill
+	A write-only single value file which exists in non-root cgroups.
+	The only allowed value is "1".
+
+	Writing "1" to the file causes the cgroup and all descendant cgroups to
+	be killed. This means that all processes located in the affected cgroup
+	tree will be killed via SIGKILL.
+
+	Killing a cgroup tree will deal with concurrent forks appropriately and
+	is protected against migrations.
+
+	In a threaded cgroup, writing this file fails with EOPNOTSUPP as
+	killing cgroups is a process directed operation, i.e. it affects
+	the whole thread-group.
+
 Controllers
 ===========