Update io_u.c comments

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index 6234c42..b8f090b 100644
--- a/io_u.c
+++ b/io_u.c
@@ -506,6 +506,9 @@
 	}
 }
 
+/*
+ * Complete a single io_u for the sync engines.
+ */
 long io_u_sync_complete(struct thread_data *td, struct io_u *io_u,
 			endio_handler *handler)
 {
@@ -521,6 +524,9 @@
 	return -1;
 }
 
+/*
+ * Called to complete min_events number of io for the async engines.
+ */
 long io_u_queued_complete(struct thread_data *td, int min_events,
 			  endio_handler *handler)