mm, docs: update memory.stat description with workingset* entries

Commit 4b4cea91691d ("mm: vmscan: fix IO/refault regression in cache
workingset transition") introduced three new entries in memory stat
file:

 - workingset_refault
 - workingset_activate
 - workingset_nodereclaim

This commit adds a corresponding description to the cgroup v2 docs.

Link: http://lkml.kernel.org/r/1494530293-31236-1-git-send-email-guro@fb.com
Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/cgroup-v2.txt

@@ -918,6 +918,18 @@ PAGE_SIZE multiple when read back.
 
 		Number of major page faults incurred
 
+	  workingset_refault
+
+		Number of refaults of previously evicted pages
+
+	  workingset_activate
+
+		Number of refaulted pages that were immediately activated
+
+	  workingset_nodereclaim
+
+		Number of times a shadow node has been reclaimed
+
   memory.swap.current
 
 	A read-only single value file which exists on non-root