Issue #3610012: Keep the settle task's wording in payment terms, not booking-specific

Follow-up to #3610001. The SettleTask and Payable are domain-neutral, but their docblocks, the cancel-when-zero form text, and the Payable docblock used booking-specific examples (a free booking, a deposit, "the order owes"). This replaces them with plain payment vocabulary (authorize, capture, cancel, capture amount, card verification), so nothing domain-specific leaks into the generic layer.

Wording only, no behavior change. Kernel tests green.