memcg: fix pgpgin/pgpgout documentation

The two memcg stats pgpgin/pgpgout have different meaning than the ones
in vmstat, which indicates that we picked a bad naming for them.

It might be late to change the stat name, but better documentation is
always helpful.

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