Update rigctl.1 man page

(cherry picked from commit 44baf44dcc)

doc/man1/rigctl.1

@@ -88,6 +88,8 @@ etc. on Linux,
 etc. on MS Windows.  The BSD flavors and Mac OS/X have their own designations.
 See your system's documentation.
 .IP
+Can be a network address:port, e.g. 127.0.0.1:12345
+.IP
 The special string \(lquh\-rig\(rq may be given to enable micro-ham device
 support.
 .