Document --udp-counters-64bit in manual page. (#796)
Fixes #791.
diff --git a/src/iperf3.1 b/src/iperf3.1
index 1ab3cd7..b9085e4 100644
--- a/src/iperf3.1
+++ b/src/iperf3.1
@@ -353,6 +353,13 @@
in a JSON object; otherwise it is appended at the bottom of the
human-readable output.
.TP
+.BR --udp-counters-64bit
+Use 64-bit counters in UDP test packets.
+The use of this option can help prevent counter overflows during long
+or high-bitrate UDP tests. Both client and server need to be running
+at least version 3.1 for this option to work. It may become the
+default behavior at some point in the future.
+.TP
.BR --repeating-payload
Use repeating pattern in payload, instead of random bytes.
The same payload is used in iperf2 (ASCII '0..9' repeating).
