feat: document invoice contract public functions

[i-amdaveee]

Summary

Closes #192

Adds /// rustdoc comments to every public entry point in both invoice contract trees:

• COMEBACKHERE-contracts/contracts/invoice/src/lib.rs
• contracts/invoice/src/lib.rs

No logic was changed — this is documentation only.

What was documented

Each public function now has:

• A summary line describing what it does
• # Parameters — every argument explained
• # Returns — where the return value is non-obvious
• # Errors — every error variant the function can return, with conditions
• # Events — the event emitted on success (where applicable)

Cross-contract calls are explicitly noted. raise_dispute documents that it calls
treasury.raise_dispute(claimant, settlement_id, reason) and that set_treasury
must be called first. set_treasury links back to raise_dispute in its doc comment.

The contract struct (InvoiceContract) also has a top-level doc comment summarising
the contract's responsibilities and the treasury dependency.

cargo doc output

Both invoice crates pass cargo doc --no-deps with zero warnings from our code.
The only warnings present are unexpected cfg condition value: testutils originating
from the soroban SDK macros themselves — these exist on main before this change.

cargo test output

cargo test fails on both this branch and main with a pre-existing
stellar-xdr/ethnum version conflict unrelated to this change:

error[E0512]: cannot transmute between types of different sizes --> ethnum-1.5.0/src/error.rs:16:14 error: could not compile ethnum (lib) due to 1 previous error error: could not compile stellar-xdr (lib) due to 1836 previous errors

Verified by running `git stash` and reproducing the identical failure on `main`
before our changes were applied.

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