cgroup: Update documentation reference

The cgroup_subsys structure references a documentation file that has been
renamed after the v1/v2 split.  Since the v2 documentation doesn't
currently contain any information on kernel interfaces for controllers,
point the user to the v1 docs.

Cc: Tejun Heo <tj@kernel.org>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>