[PATCH] tiny kmem_cache_destroy doc tweak

From:  John Levon <levon@movementarian.org>

  Make it clear that _destroy() won't do the kmem_cache_free()s for you

mm/slab.c

@@ -1191,6 +1191,8 @@ int kmem_cache_shrink(kmem_cache_t *cachep)
  * cache being allocated each time a module is loaded and unloaded, if the
  * module doesn't have persistent in-kernel storage across loads and unloads.
  *
+ * The cache must be empty before calling this function.
+ *
  * The caller must guarantee that noone will allocate memory from the cache
  * during the kmem_cache_destroy().
  */