cgroup: update sane_behavior documentation

f12dc020 ("cgroup: mark "tasks" cgroup file as insane") and
cc5943a7 ("cgroup: mark "notify_on_release" and "release_agent"
cgroup files insane") forgot to update the changed behavior
documentation in cgroup.h.  Update it.

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