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/net.c b/engines/net.c
index cd1fda9..51bc671 100644
--- a/engines/net.c
+++ b/engines/net.c
@@ -1,5 +1,8 @@
 /*
- * Transfer data over the net.
+ * net engine
+ *
+ * IO engine that reads/writes to/from sockets.
+ *
  */
 #include <stdio.h>
 #include <stdlib.h>