Update io engine comments

Some were plain wrong, mainly because they were born through
copying an existing engine.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/engines/sync.c b/engines/sync.c
index b1776d2..a137282 100644
--- a/engines/sync.c
+++ b/engines/sync.c
@@ -1,5 +1,8 @@
 /*
- * regular read/write sync io engine
+ * sync engine
+ *
+ * IO engine that does regular read(2)/write(2) with lseek(2) to transfer
+ * data.
  *
  */
 #include <stdio.h>