Document pvsync engine

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/fio.1 b/fio.1
index 5082bf8..f54ea34 100644
--- a/fio.1
+++ b/fio.1
@@ -448,6 +448,9 @@
 Basic \fIreadv\fR\|(2) or \fIwritev\fR\|(2) I/O. Will emulate queuing by
 coalescing adjacents IOs into a single submission.
 .TP
+.B pvsync
+Basic \fIpreadv\fR\|(2) or \fIpwritev\fR\|(2) I/O.
+.TP
 .B libaio
 Linux native asynchronous I/O. This ioengine defines engine specific options.
 .TP