Arvados

See #18790. Currently the only documentation is arvados-client logs -help:

Usage: arvados-client logs [options] container-request-uuid
  -f    follow: poll for new data until the container finishes
  -poll duration
        minimum duration to wait before polling for new data (default 2s)

This probably deserves to be mentioned at appropriate points in the user guide.

This might be a bit of a yak-shaving exercise. Currently the arvados-client program isn't even mentioned in the CLI installation / usage docs.