cgroup: docs: add pswpin and pswpout items in cgroup v2 doc

The commit 15ff4d4 ("mm/memcontrol: add per-memcg pgpgin/pswpin
counter") introduced the pswpin and pswpout items in the memory.stat of
cgroup v2.  Therefore, update them accordingly in the cgroup-v2
documentation.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 15ff4d4 ("mm/memcontrol: add per-memcg pgpgin/pswpin counter") 
Signed-off-by: Hao Jia <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Michal Koutný <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Shakeel Butt <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Hao Jia

Documentation/admin-guide/cgroup-v2.rst

@@ -1561,6 +1561,12 @@ The following nested keys are defined.
 	  workingset_nodereclaim
 		Number of times a shadow node has been reclaimed
 
+	  pswpin (npn)
+		Number of pages swapped into memory
+
+	  pswpout (npn)
+		Number of pages swapped out of memory
+
 	  pgscan (npn)
 		Amount of scanned pages (in an inactive LRU list)