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/mmap.c b/engines/mmap.c
index 0922e97..17cf5fc 100644
--- a/engines/mmap.c
+++ b/engines/mmap.c
@@ -1,5 +1,8 @@
 /*
- * regular read/write sync io engine
+ * mmap engine
+ *
+ * IO engine that reads/writes from files by doing memcpy to/from
+ * a memory mapped region of the file.
  *
  */
 #include <stdio.h>