slab: make kmem_cache_create{_usercopy} description proper kernel-doc

Add the description for kmem_cache_create, fixup the return value paragraph
and make both kmem_cache_create and add the second '*' to the comment
opening.

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