cgroup: Update remount documentation

Remounting support will be removed in the future (see
feature-removal-schedule.txt).  Discourage users from using it.

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Rob Landley <rob@landley.net>
Cc: Paul Menage <paul@paulmenage.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>