auth/azure: Documents config env vars and fixes resource used in examples

[austingebauer]

This PR improves the Azure auth documentation by:

1. Documenting environment variables that can be used to configure the auth method.
2. Fixing the resource parameter that's used in examples for requesting a JWT access token from the instance metadata service. The current example doesn't work and had caused some confusion. Using the Azure resource manager endpoint aligns with defaults in our Azure auth client library and examples.
3. Providing additional explanation for the jwt and resource parameters