memcg: document cgroup dirty/writeback memory statistics

Signed-off-by: Sha Zhengju <[e-mail]>
Cc: Fengguang Wu <[e-mail]>
Cc: Greg Thelen <[e-mail]>
Cc: KAMEZAWA Hiroyuki <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/cgroups/memory.txt

···
 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
+writeback	- # of bytes of file/anon cache that are queued for syncing to
+		disk.
 inactive_anon	- # of bytes of anonymous and swap cache memory on inactive
 		LRU list.
 active_anon	- # of bytes of anonymous and swap cache memory on active