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/sg.c b/engines/sg.c
index 98ecbd1..84c2afe 100644
--- a/engines/sg.c
+++ b/engines/sg.c
@@ -1,5 +1,7 @@
 /*
- * scsi generic sg v3 io engine
+ * sg engine
+ *
+ * IO engine that uses the Linux SG v3 interface to talk to SCSI devices
  *
  */
 #include <stdio.h>