chore: move acir docs to code declaration (#5040)

# Description

## Problem\*

Resolves a few minor inaccuracies in the ACIR docs and moves their
primary location to Rust.

## Summary\*

ACIR docs get stale quickly when detached from code/tests. This PR moves
them to the code for now; we expect to move them back once ACIR is more
stable.

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

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Co-authored-by: Tom French <[email protected]>
Co-authored-by: Tom French <[email protected]>
Co-authored-by: Maxim Vezenov <[email protected]>