BGP: Add documentaion for extended messages

doc/bird.sgml

@@ -1862,6 +1862,11 @@ using the following configuration parameters:
 	in neighbor's implementation of 4B AS extension. Even when disabled
 	(off), BIRD behaves internally as AS4-aware BGP router. Default: on.
 
+	<tag>enable extended messages <m/switch/</tag>
+	The BGP protocol uses maximum message length of 4096 bytes. This option
+	provides an extension to allow extended messages with length up
+	to 65535 bytes. Default: off.
+
 	<tag>capabilities <m/switch/</tag>
 	Use capability advertisement to advertise optional capabilities. This is
 	standard behavior for newer BGP implementations, but there might be some

nest/config.Y

@@ -111,7 +111,7 @@ idval:
        $$ = ipa_to_u32(SYM_VAL($1).px.ip);
 #endif
      else
-       cf_error("Number of IPv4 address constant expected");
+       cf_error("Number or IPv4 address constant expected");
    }
  ;