Updating docs for app_name on ConfigLoader (#3645)

## 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 -->
Docs indicating that the `AppName` can be loaded from an environment
variable or profile key were buried in the default provider and would be
hard to find for users.

## Description
<!--- Describe your changes in detail -->
This change adds documentation to the `app_name` method on
`ConfigLoader` that makes the options for loading the `AppName` more
explicit.

## 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. -->
Ran `cargo doc` to ensure the docs were generated correctly

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
----

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Co-authored-by: ysaito1001 <awsaito@amazon.com>