cgroup: Add documentation for missing zswap memory.stat

This includes zswpin, zswpout and zswpwb.

Signed-off-by: Usama Arif <usamaarif642@gmail.com>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20240502185307.3942173-2-usamaarif642@gmail.com>

Author: uarif1

Documentation/admin-guide/cgroup-v2.rst

@@ -1572,6 +1572,15 @@ PAGE_SIZE multiple when read back.
 	  pglazyfreed (npn)
 		Amount of reclaimed lazyfree pages
 
+	  zswpin
+		Number of pages moved in to memory from zswap.
+
+	  zswpout
+		Number of pages moved out of memory to zswap.
+
+	  zswpwb
+		Number of pages written from zswap to swap.
+
 	  thp_fault_alloc (npn)
 		Number of transparent hugepages which were allocated to satisfy
 		a page fault. This counter is not present when CONFIG_TRANSPARENT_HUGEPAGE