Add note in man page about the -w flag getting transmitted to the server.

src/iperf3.1

@@ -77,7 +77,7 @@ number of parallel client streams to run
 run in reverse mode (server sends, client receives)
 .TP
 .BR -w ", " --window " \fIn\fR[KM]"
-TCP window size (socket buffer size)
+TCP window size / socket buffer size (this gets sent to the server and used on that side too)
 .TP
 .BR -B ", " --bind " \fIn\fR[KM]"
 bind to a specific interface or multicast address