Consider Adding Documentation Describing Dynamic Modification of Terraform Configuration

### Does this documentation exist?

* [ ] This is new documentation
* [x] This is an enhancement to existing documentation

### Where would you expect to find this documentation?

* [x] On terraform.io
* [ ] In the GoDoc for this module
* [ ] In this repo as a markdown file
* [ ] Somewhere else

#### Details

Recent changes to `terraform-plugin-testing` (i.e., [Add terraform block to config when not already present](https://github.com/hashicorp/terraform-plugin-testing/pull/216#top) released in [v1.6.0](https://github.com/hashicorp/terraform-plugin-testing/releases/tag/v1.6.0)) result in the dynamic addition of a `terraform` block to configuration supplied in the `TestStep.Config` field, to explicitly specify `required_providers`.

### Description

Consider adding documentation to describe and illustrate how the supplied terraform configuration can be dynamically modified during the execution of acceptance tests.

### References

- #264