Add better support for reporting training continuation values

[gregtatum]

This makes the step to kick off training a bit nicer for training continuation. Previously it was one line and hard to read.

       experiment.name: decoder-tiny
        experiment.src: en
        experiment.trg: lt
           start-stage: train-student
          target-stage: all-pipeline
    previous_group_ids: ['YxrSV7eHR62FHcMmWmsZ6w', 'LtiIriAwSpeT1-GCYqd9Iw', 'e6HrD2nFT8S-Hckt18pxlQ', 'WDNdtYFhTG6gh7c1wnAEyw']
         train-teacher: {
                          "urls": [
                            "https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/FmL_NEMqQ9qHVZd9tdvbfw/artifacts/public/build/",
                            "https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/biiW0pGaQ_uWBw-1GLwDgA/artifacts/public/build/"
                          ],
                          "mode": "use",
                          "type": "default"
                        }

I also clarified the docs, as it took me a bit to verify I had the URL correct.