Clarify Use of `azureml.core.runconfig.DockerConfiguration`

@keijik @cody-dkdc

[In this example notebook](https://github.com/Azure/MachineLearningNotebooks/blob/master/how-to-use-azureml/ml-frameworks/fastai/fastai-with-custom-docker/fastai-with-custom-docker.ipynb), you are using deprecated code

```py
from azureml.core import Environment

fastai_env = Environment("fastai")
fastai_env.docker.enabled = True
```
I get the warning

```
'enabled' is deprecated. Please use the azureml.core.runconfig.DockerConfiguration object with the 'use_docker' param instead.
```

However, from [the docs](https://docs.microsoft.com/en-us/python/api/azureml-core/azureml.core.runconfig.dockerconfiguration?view=azure-ml-py) its a bit ambiguous on how to use it. It appears that the below code is incorrect. What is the correct way to use this instead? Can the documentation be improved? Also can you provide a better warning stating the version number at which this will be deprecated, or say that it will be deprecated in the future since it isn't deprecated as of right now?

```py
from azureml.core.runconfig import DockerConfiguration
dc = DockerConfiguration(use_docker=True)
fastai_env.docker = dc
```

The [documentation for this](https://docs.microsoft.com/en-us/python/api/azureml-core/azureml.core.runconfig.dockerconfiguration?view=azure-ml-py) doesn't really provide many hints on where to use this configuration correctly. Can an example be made for this?