Improve documentation on additional role assigments and examples

**Describe the solution you'd like**

The documentation for adding additional role requirements at https://github.com/Azure/data-management-zone/blob/main/docs/EnterpriseScaleAnalytics-ServicePrincipal.md#adding-additional-role-assigments
could be improved by adding additional text to describe that the role assignments should be done on either resource or resource group level. Also clear examples in the code examples with values makes it easier to follow and understand.

See discussion with marvibuss here https://github.com/Azure/data-management-zone/discussions/199