Update atlas mongos configuration docs

https://docs.atlas.mongodb.com/create-new-cluster/#scale-your-replica-set-to-a-sharded-cluster

Atlas deploys six routers (mongos programs) for a sharded cluster. Atlas runs the routers on the shard servers. For cross-region clusters, Atlas spreads the routers across the selected regions.

This is no longer true. Atlas deploys a mongos on every shard server. This allows us to have geo-affinity to mongos from user applications from whichever region they are in.

When connecting from a driver, all mongos are included in the connection string. When connecting from the shell, only the first mongos is returned displayed.

cc: cory.mintz