For sharded clusters, the addUser() command needs to be run on the primary for each shard and on the mongos. The doc currently says:

MongoDB Deployment: in a mongo shell connected to either the replica set primary for replica sets, or a mongos for sharded clusters.

I think we need to be more clear about this. The way I'm reading it, it sounds like you only need to run it for mongos for sharded clusters.

This is potentially confusing, since for most user administration purposes adding the user via mongos is sufficient. This is a special case since the MMS agent connects directly to mongod.