Improve docs no credentials (#3279)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
- aws-sdk-rust#971

## Description
Add documentation to `no_credentials` and add `test_credentials()`
method

## Testing
CI

## Checklist
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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Co-authored-by: John DiSanti <jdisanti@amazon.com>