Fix generated summary docs for convenience HashMap / Vec members (#2914)

Previously the docs said `Vec<String>` when the method actually accepted
`String`, similarly for HashMap.

## Motivation and Context
- https://github.com/awslabs/aws-sdk-rust/issues/825

## Description
Fix the generator to be aware of Vec/Hashmap methods

## Testing
- UT
- [x] audit codegen diff

## 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 AWS
SDK, generated SDK code, or SDK runtime crates

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