Add MsiAuthentication to signature for Workspace() auth parameter

For the Workspace() auth parameter, I believe MsiAuthentication() is also supported, there are examples of its usage in MS docs.

---
#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: 90efe042-27ac-6e77-9292-10df77bb1c36
* Version Independent ID: 3b9f2576-7d3f-00f8-0afe-549960520f08
* Content: [azureml.core.Workspace class - Azure Machine Learning Python](https://docs.microsoft.com/en-us/python/api/azureml-core/azureml.core.workspace(class)?view=azure-ml-py)
* Content Source: [AzureML-Docset/stable/docs-ref-autogen/azureml-core/azureml.core.Workspace(class).yml](https://github.com/MicrosoftDocs/MachineLearning-Python-pr/blob/live/AzureML-Docset/stable/docs-ref-autogen/azureml-core/azureml.core.Workspace(class).yml)
* Service: **machine-learning**
* Sub-service: **core**
* GitHub Login: @debfro
* Microsoft Alias: **jmartens**