mm/slub: make the description of slab_min_objects helpful in doc

There is no a value assigned to slab_min_objects by default, it always
is 0 that is initialized by compiler if no assigned value by command line.
min_objects is calculated based on processor numbers in calculate_order().
For more details, see commit 9b2cd506 ("slub: Calculate min_objects
based on number of processors.")

Signed-off-by: Xiongwei Song <xiongwei.song@windriver.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>