cgroup: Miscellaneous cgroup documentation.

Documentation of miscellaneous cgroup controller. This new controller is
used to track and limit the usage of scalar resources.

Signed-off-by: Vipin Sharma <vipinsh@google.com>
Reviewed-by: David Rientjes <rientjes@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>