cgroup: "cgroup.subtree_control" should be writeable by delegatee

"cgroup.subtree_control" determines which resource types a cgroup
wants to control.  Unlike actual resource knobs, this is an attribute
which belongs to the cgroup itself instead of its parent and thus
should be writeable by the delegatee in a delegated cgroup.

Update delegation documentation accordingly.

Signed-off-by: Tejun Heo <tj@kernel.org>