mm: memcg: add reminder comment for the memcg v2 events

To maintain the correct state, it is important to ensure that events for
the memory cgroup v2 are aligned with the sample cgroup codes.

Link: https://lkml.kernel.org/r/20231123071945.25811-4-ddrokosov@salutedevices.com


Signed-off-by: Dmitry Rokosov <ddrokosov@salutedevices.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>