Default single-node connstrs to direct connection; improve docs

[FGasper]

mongosh defaults single-node connections to a direct connection. This changeset applies that logic to migration-verifier so that, if a connection string includes only a single node, the verifier will always connect to it.

This is useful, e.g., when using a single, containerized mongod for the metadata DB and the mongod’s replication is set up to reference its hostname. In that case a direct connection is necessary when using Podman or other container engines that don’t “inject” the container name into the host’s name resolution (as, e.g., Docker Desktop on macOS does).

At the same time, this changeset avoids the above problem by explicitly making the documentation’s container one-liner bind to localhost.

[tdq45gj]

LGTM