Generate better docs when fields are required (#2996)

## Motivation and Context
With required builders, builders return errors in more cases now. This
clearly indicates which fields that the service considers required.

## Description
Add more docs for required status.
<img width="1037" alt="Screenshot 2023-09-22 at 3 38 42 PM"
src="https://github.com/awslabs/smithy-rs/assets/492903/082b4950-7edd-4f25-9081-a3c9fe3f870a">
<img width="998" alt="Screenshot 2023-09-22 at 3 39 01 PM"
src="https://github.com/awslabs/smithy-rs/assets/492903/98acd359-5572-4e01-afba-7af5b0264d18">

## 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. -->

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

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._