docs: field-level doc-comments for InvoiceExt struct (#700)

[gabrielujelistic-collab]

Add field-level doc-comments to InvoiceExt

Summary

Closes #700

InvoiceExt (contracts/split/src/types.rs) is the primary extension storage
struct for an invoice and holds ~40 optional/extension fields, none of which had
doc comments. This PR adds a concise one-line /// doc comment to every
field of InvoiceExt, explaining what the field stores and when it is used.
This makes the on-chain storage layout self-documenting for contributors
extending invoices (see also the new "Adding Invoice Options" section in
CONTRIBUTING.md).

What changed

Added field-level documentation for: co_signers, required_signatures,
signatures, approver, approved, condition_oracle, condition_met,
penalty_bps, penalty_deadline, min_funding_bps, release_stages,
released_stages, allowed_payers, price_oracle, base_amounts,
swap_tokens, tax_bps, tax_authority, insurance_premium_bps,
insurance_fund, smart_route, convert_to_stream, accepted_tokens,
forward_to, forward_invoice_id, split_rules, auto_resolve_rules,
creator_cosigner, velocity_limit, velocity_window, parent_invoice_id,
pause_reason, auto_resume_at, payment_cooldown_secs,
max_payments_per_window, payment_window_secs, scheduled_release_at,
penalty_tiers, allowed_callers, refund_grace_secs.

No field names, types, or ordering changed — this is a documentation-only,
non-breaking change.

Verification

Documentation-only; compiles unaffected by doc comments. (As with the other
split PRs, cargo check on main currently fails for unrelated, pre-existing
reasons — unimplemented helpers/variants from other open issues — so CI red here
is not introduced by this change.)

closes #700

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