Documentation: memcg: future proof hierarchical statistics documentation

The hierarchical versions of per-memcg counters in memory.stat are all
calculated the same way and are all named total_<counter>.

Documenting the pattern is easier for maintenance than listing each
counter twice.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Ying Han <yinghan@google.com>
Randy Dunlap <rdunlap@xenotime.net>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>