Presence of multiple similar environment variables is confusing

### Describe the feature

In documentation, deprecate any environment variables that enable duplicate functionality. Deprecate means that it's still fully supported by the CLI and will continue to be, but it's not surfaced in the same way in documentation.

### Use Case

For reasons that are a little unclear, the CLI supports both `AWS_DEFAULT_REGION` and `AWS_REGION`. Both of these are [documented](https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-envvars.html), but it's not clear why I would want to use one over another. It seems that `AWS_DEFAULT_REGION` is overridden by `AWS_REGION`. Would it make sense to standardize behavior across the various SDKs and deprecate `AWS_DEFAULT_REGION` - as well as any other similar variables?

### Proposed Solution

I suggest moving deprecated environment variables to the bottom of the documentation with a note that they are included for backwards compatibility. Otherwise, please document the difference between environment variables and why I would want to use one over another.

### Other Information

This feature request is the result of an internal Slack discussion.

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CLI version used

ALL

### Environment details (OS name and version, etc.)

ALL