BGP: Add documentaion for extended messages

	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

       $$ = 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");

   }

 ;