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>

Documentation/cgroups/memory.txt

@@ -410,8 +410,11 @@ memory.stat file includes following statistics
 cache		- # of bytes of page cache memory.
 rss		- # of bytes of anonymous and swap cache memory.
 mapped_file	- # of bytes of mapped file (includes tmpfs/shmem)
-pgpgin		- # of pages paged in (equivalent to # of charging events).
-pgpgout		- # of pages paged out (equivalent to # of uncharging events).
+pgpgin		- # of charging events to the memory cgroup. The charging
+		event happens each time a page is accounted as either mapped
+		anon page(RSS) or cache page(Page Cache) to the cgroup.
+pgpgout		- # of uncharging events to the memory cgroup. The uncharging
+		event happens each time a page is unaccounted from the cgroup.
 swap		- # of bytes of swap usage
 inactive_anon	- # of bytes of anonymous memory and swap cache memory on
 		LRU list.