DOC: Put some information about "bindingip" parameter in the documentation around XDCR

When connection Couchbase to Elasticsearch Couchbase plugin we are using the hostname/IP address of the machine, but Elasticsearch send back the "binding ip" from the VM that is not necessary the same and more importantly this could change on non stable network. (this happen in development mode, demonstration, conference...)

It will be interesting to point in our document that is possible to force the IP/Hostname from Elasticsearch using the network.bind_host parameter.

See http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/modules-network.html