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>

Documentation/core-api/mm-api.rst

@@ -46,6 +46,9 @@ The Slab Cache
 .. kernel-doc:: mm/slab.c
    :export:
 
+.. kernel-doc:: mm/slab_common.c
+   :export:
+
 .. kernel-doc:: mm/util.c
    :functions: kfree_const kvmalloc_node kvfree