BGP attribute ext-next-hop

Hi all, The version of my bird is 2.0.0, I found the source code of bird can support the attribute ext_next_hop for BGP, and this attribute is also parsed in cf-parse.y, but cannot found in user's guide document. Does anyone know what configuration is coherent with the attribute ext_next_hop? Is the attribute ext_next_hop fully supported in version 2.0.0 ? Thanks Arvin