Client.scheduler_info needs better documentation

https://distributed.readthedocs.io/en/latest/api.html#distributed.Client.scheduler_info provides a single example of the dictionary returned, but there's no explanation of what those fields mean. Take for example `'stored': 0` under the worker -- the example alone doesn't do enough to explain what that actually means. A list of the fields being returned and their interpretations would go a long way.