Documentation/cgroup-v1: fix outdated programming details

The cgroup-v1 documentation is out of date in a few places: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: NMatt Roper <matthew.d.roper@intel.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

Documentation/cgroup-v1/cgroups.txt

@@ -523,12 +523,7 @@ Accessing a task's cgroup pointer may be done in the following ways:
 Each subsystem should:
 
 - add an entry in linux/cgroup_subsys.h
-- define a cgroup_subsys object called <name>_subsys
-
-If a subsystem can be compiled as a module, it should also have in its
-module initcall a call to cgroup_load_subsys(), and in its exitcall a
-call to cgroup_unload_subsys(). It should also set its_subsys.module =
-THIS_MODULE in its .c file.
+- define a cgroup_subsys object called <name>_cgrp_subsys
 
 Each subsystem may export the following methods. The only mandatory
 methods are css_alloc/free. Any others that are null are presumed to