Probably more clarified.

fixes #1245

samples/knot.full.conf

@@ -47,7 +47,7 @@ interfaces {
   # Format 1: <name> { address <address>; [port <port>;] }
   ipv4 {                # <name> is an arbitrary symbolic name
     address 127.0.0.1;  # <address> may be ither IPv4 or IPv6 address
-    port 53531;         # port is optional (default: 53)
+    port 53531;         # port is required for XFR/IN and NOTIFY/OUT 
   }
 
   # Format 2: <name> { address <address>@<port>; }