Update docs for non aws-config config builders (#3960)

## 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 -->
Partially addresses https://github.com/awslabs/aws-sdk-rust/issues/1226,
although we might want to look into going further and actually
deprecating or warning on these builder methods.

## Description
<!--- Describe your changes in detail -->

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
Doc only update

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