code: func (d *Disco) CredentialsForHost(hostname svchost.Hostname) (auth.HostCredentials, error) {
  	if d.credsSrc == nil {
  		return nil, nil
  	}
  	return d.credsSrc.ForHost(hostname)
  }
docs: // CredentialsForHost returns a non-nil HostCredentials if the embedded source has
  // credentials available for the host, and a nil HostCredentials if it does not.