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/20250318075833.90615-3-jiahao.kernel@gmail.com
Fixes: 15ff4d4 ("mm/memcontrol: add per-memcg pgpgin/pswpin counter") 
Signed-off-by: Hao Jia <jiahao1@lixiang.com>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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)