Documentation: cgroup-v2: add memory.stat::sock description

Signed-off-by: NJohannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov@virtuozzo.com>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/cgroup-v2.txt

@@ -843,6 +843,10 @@ PAGE_SIZE multiple when read back.
 		Amount of memory used to cache filesystem data,
 		including tmpfs and shared memory.
 
+	  sock
+
+		Amount of memory used in network transmission buffers
+
 	  file_mapped
 
 		Amount of cached filesystem data mapped with mmap()