memcg: update documentation to describe usage_in_bytes

Since 569b846d ("memcg: coalesce uncharge during unmap/truncate"), we do
batched (delayed) uncharge at truncation/unmap.  And since cdec2e42(memcg:
coalesce charging via percpu storage), we have percpu cache for
res_counter.

These changes improved performance of memory cgroup very much, but made
res_counter->usage usually have a bigger value than the actual value of
memory usage.  So, *.usage_in_bytes, which show res_counter->usage, are
not desirable for precise values of memory(and swap) usage anymore.

Instead of removing these files completely(because we cannot know
res_counter->usage without them), this patch updates the meaning of those
files.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>