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/cpu.c b/engines/cpu.c
index b6f9075..d10669d 100644
--- a/engines/cpu.c
+++ b/engines/cpu.c
@@ -1,3 +1,10 @@
+/*
+ * CPU engine
+ *
+ * Doesn't transfer any data, merely burns CPU cycles according to
+ * the settings.
+ *
+ */
 #include "../fio.h"
 #include "../os.h"