slub: add Documentation/ABI/testing/sysfs-kernel-slab

Adds documentation for the slub ABI.

This is placed in the `testing' directory since the meanings of these
files are still subject to change as slub is developed.

Acked-by: Christoph Lameter <cl@linux-foundation.org>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>