cgroup: clean up MAINTAINERS entries

There are currently three cgroup related entries in MAINTAINERS.  Make
the following updates.

* Make the names - both cgroup and cpuset - singular.  We're mixing
  singular and plural all over the place for no good reason.

* Drop containers@lists.linux-foundation.org from CGROUP.  That list
  doesn't have much to do with cgroup per-se.

* Add Documentation field to CGROUP.

* Drop mm/*cgroup* from CGROUP.  memcg has separate maintainers.

* Prefix the controller-specific ones with "CONTROL CGROUP -" and
  collect cgroup related entries under the core one.

* Add (MEMCG) abbreviation to MEMCG entry.

* Drop Balbir Singh and KAMEZAWA Hiroyuki from memcg maintainers.  It
  has been quite a while since both actually worked on memcg.

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