docs/mm-api: link slab_common.c to "The Slab Cache" section

Several functions in mm/slab_common.c have kernel-doc comments, it makes
perfect sense to link them to the MM API reference.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>