[DOCUMENTATION] Add inline doc comments to all Rust contract functions

[dev-fatima-24]

Priority: medium

Description

The Soroban contract functions in all four contracts have no doc comments, making it difficult for auditors and contributors to understand the business logic. Every public function must have a Rust doc comment explaining its purpose, parameters, return value, and which errors it can return.

Acceptance Criteria

• Every pub fn in all four contracts has a /// doc comment
• Comments explain the business purpose, not just the technical mechanics
• Parameters and return values are documented
• Possible error variants (from CarbonError) are listed with conditions
• cargo doc generates valid HTML documentation without warnings

[raphael-01-visual]

@raphael-01-visual has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Can I work on this issue ? ETA - 12hrs

ℹ️ Repo Maintainers: To accept this application, review their application or assign @raphael-01-visual to this issue.

[drips-wave]

Congratulations, @raphael-01-visual! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @raphael-01-visual: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊