Async verify HOWTO/man update

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index 55662d3..5099c83 100644
--- a/HOWTO
+++ b/HOWTO
@@ -832,7 +832,10 @@
 		thread. This option takes an integer describing how many
 		async offload threads to create for IO verification instead,
 		causing fio to offload the duty of verifying IO contents
-		to one or more separate threads.
+		to one or more separate threads. If using this offload
+		option, even sync IO engines can benefit from using an
+		iodepth setting higher than 1, as it allows them to have
+		IO in flight while verifies are running.
 
 verify_async_cpus=str	Tell fio to set the given CPU affinity on the
 		async IO verification threads. See cpus_allowed for the