slab, documentation: add description of debugfs files for SLUB caches

Add description of debugfs files alloc_traces and free_traces
to SLUB cache documentation.

[ vbabka@suse.cz: some rewording ]

Signed-off-by: Oliver Glitta <glittao@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-doc@vger.kernel.org