peering: document that the unconfirmed-settlement cursor can skip invoices until it resets #1135

Follow‑up from the re‑review of #1087 (N3). Not a defect, a correctness caveat that readers will otherwise misread.

Problem

The maintenance cursor advances a full page whenever the page is full (src/app/maintenance-worker.ts:218). Successful retries shrink the result set, so an invoice can shift into the window the cursor has already passed and will not be revisited until a short page resets the offset to 0. That is a delay, never a loss, so the current behavior is acceptable.

The risk is expectation: someone reading the code will assume the cursor is an exact resume point and build on that assumption.