ABI: sysfs-kernel-slab: Document some stats

Document three slab stats that were added in 2011 and 2017.

Fixes: 49e22585 ("slub: per cpu cache for partial pages")
Fixes: 8eb8284b ("usercopy: Prepare for usercopy whitelisting")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/8960435734c5aa918568ce7891171448c4d68709.1632750608.git.mchehab+huawei@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-kernel-slab

@@ -496,3 +496,24 @@ Contact:	Pekka Enberg <penberg@cs.helsinki.fi>,
 Description:
 		Writing to the validate file causes SLUB to traverse all of its
 		cache's objects and check the validity of metadata.
+
+What:		/sys/kernel/slab/<cache>/usersize
+Date:		Jun 2017
+Contact:	David Windsor <dave@nullcore.net>
+Description:
+		The usersize file is read-only and contains the usercopy
+		region size.
+
+What:		/sys/kernel/slab/<cache>/slabs_cpu_partial
+Date:		Aug 2011
+Contact:	Christoph Lameter <cl@linux.com>
+Description:
+		This read-only file shows the number of partialli allocated
+		frozen slabs.
+
+What:		/sys/kernel/slab/<cache>/cpu_partial
+Date:		Aug 2011
+Contact:	Christoph Lameter <cl@linux.com>
+Description:
+		This read-only file shows the number of per cpu partial
+		pages to keep around.