udp: mention default in documentation

Signed-off-by: rogerdpack <rogerpack2005@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/protocols.texi

@@ -1102,6 +1102,7 @@ The list of supported options follows.
 @item buffer_size=@var{size}
 Set the UDP maximum socket buffer size in bytes. This is used to set either
 the receive or send buffer size, depending on what the socket is used for.
+Default is 64KB.  See also @var{fifo_size}.
 
 @item localport=@var{port}
 Override the local UDP port to bind with.