Documentation/cgroup-v1: fix outdated programming details

The cgroup-v1 documentation is out of date in a few places:

 * cgroup controllers can no longer be compiled as modules since commit
   3ed80a62 ("cgroup: drop module support"); the functions and fields
   referenced here no longer exist.

 * Controllers need to create of a cgroup_subsys object named
   "<name>_cgrp_subsys" instead of "<name>_subsys" since commit
   073219e9 ("cgroup: clean up cgroup_subsys names and initialization")

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