Remove doc hidden from struct fields (#2854)

## Motivation and Context
Removes `#[doc(hidden)]` from struct fields.

## Testing
- [ ] Passed tests in CI

## Checklist
- [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>