docs/core-api: memory-allocation: add mention of kmem_cache_create_userspace

Mention that when a part of a slab cache might be exported to the
userspace, the cache should be created using kmem_cache_create_usercopy()

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