Document the ioengine=net pingpong= option

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/fio.1 b/fio.1
index 26a4650..b7abc4b 100644
--- a/fio.1
+++ b/fio.1
@@ -1162,6 +1162,15 @@
 connections rather than initiating an outgoing connection. The
 hostname must be omitted if this option is used.
 .TP
+.BI (net, pingpong) \fR=\fPbool
+Normal a network writer will just continue writing data, and a network reader
+will just consume packages. If pingpong=1 is set, a writer will send its normal
+payload to the reader, then wait for the reader to send the same payload back.
+This allows fio to measure network latencies. The submission and completion
+latencies then measure local time spent sending or receiving, and the
+completion latency measures how long it took for the other end to receive and
+send back.
+.TP
 .BI (e4defrag,donorname) \fR=\fPstr
 File will be used as a block donor (swap extents between files)
 .TP