Whoops. Forgot to document the -d flag properly. Do this.

src/iperf3.1

@@ -52,6 +52,10 @@ give more detailed output
 .BR -J ", " --json " "
 output in JSON format
 .TP
+.BR -d ", " --debug " "
+emit debugging output.
+Primarily (perhaps exclusively) of use to developers.
+.TP
 .BR -v ", " --version " "
 show version information and quit
 .TP

src/locale.c

@@ -78,6 +78,7 @@ const char usage_longstr[] = "Usage: iperf [-s|-c host] [options]\n"
 #endif
                            "  -V, --verbose             more detailed output\n"
                            "  -J, --json                output in JSON format\n"
+                           "  -d, --debug               emit debugging output\n"
                            "  -v, --version             show version information and quit\n"
                            "  -h, --help                show this message and quit\n"
                            "Server specific:\n"