RE: https://docs.mongodb.com/master/core/replica-set-write-concern/

This page starts of by stating:

From the perspective of a client application, whether a MongoDB instance is running as a single server (i.e. “standalone”) or a replica set is transparent. However, replica sets offer some configuration options for write.

With the introduction of change streams in 3.6 this is no longer the case and doesn't really add much to a readers understanding of write concern.