blk-mq: Document the blk_mq_hw_queue_to_node() arguments

Document the meaning of the blk_mq_hw_queue_to_node() arguments.
Reviewed-by: Chaitanya Kulkarni <chiatanya.kulkarni@wdc.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-cpumap.c

@@ -60,7 +60,11 @@ int blk_mq_map_queues(struct blk_mq_queue_map *qmap)
 }
 EXPORT_SYMBOL_GPL(blk_mq_map_queues);
 
-/*
+/**
+ * blk_mq_hw_queue_to_node - Look up the memory node for a hardware queue index
+ * @qmap: CPU to hardware queue map.
+ * @index: hardware queue index.
+ *
  * We have no quick way of doing reverse lookups. This is only used at
  * queue init time, so runtime isn't important.
  */