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.comSigned-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>

mm/memcontrol.c

@@ -6783,6 +6783,10 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
 	return nbytes;
 }
 
+/*
+ * Note: don't forget to update the 'samples/cgroup/memcg_event_listener'
+ * if any new events become available.
+ */
 static void __memory_events_show(struct seq_file *m, atomic_long_t *events)
 {
 	seq_printf(m, "low %lu\n", atomic_long_read(&events[MEMCG_LOW]));