docs: field-level doc-comments for InvoiceOptions struct (#701)

[gabrielujelistic-collab]

Add field-level doc-comments to InvoiceOptions

Summary

Closes #701

InvoiceOptions (contracts/split/src/types.rs) is the public input struct
callers fill in for create_invoice. Most fields already carried /// Issue #NN
notes, but four never-documented fields were left without any explanation. This
PR adds concise /// doc comments to those four fields: co_creators,
allow_early_withdrawal, bonus_pool, and bonus_max_payers.

What changed

• co_creators — additional creators who share ownership/rights over the invoice.
• allow_early_withdrawal — when true, payers may withdraw before the deadline.
• bonus_pool — size of the bonus pool funded alongside the invoice.
• bonus_max_payers — max distinct payers that may fund the bonus pool.

All other fields are unchanged (their existing issue-referenced comments remain).

Verification

Documentation-only; compiles unaffected. (CI on main is red for unrelated,
pre-existing reasons documented in the other split PRs.)

closes #701

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