I’m not the authority on XDCR. A mesh is implied in the section on bidirectional XDCR. Cross Data Center Replication (XDCR) | Couchbase Docs. Just as bidirectional XDCR is simply two unidirectional XDCRs, a mesh is multiple bidirectional XDCRs. There are other configurations that will propagate changes among multiple clusters - such as a ring or a star.
Here’s a white paper on Couchbase High Availability.
https://info.couchbase.com/rs/302-GJY-034/images/High_Availability_Disaster_Recovery_Globally_Distributed_Data_XDCR_.pdf