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>