docs: explain the purpose of AVS on the Billing object

[ecunado]

Summary

• Removes the plumbing description from the Billing object ("When present, it is forwarded to the payment terminal as-is and included by the terminal in the authorization request sent to the acquirer. When omitted and AVS is enabled on the device's configuration template, the terminal prompts the operator for the billing details instead.").
• Replaces it with what AVS is actually for: improving the approval rate of MOTO operations, since the card is never seen by the merchant and a matching billing address is one of the strongest signals the issuer has.
• Adds the coverage caveat (support depends on issuer and country; postal code is the most widely supported check, street address mostly US/CA/UK), which explains why zipCode is required and address is optional.
• Applies the same purpose clause to the billing row of the Transaction Request Object so both places agree.

Context

• Framing follows how AVS is normally documented in the industry — Stripe describes it as a card-not-present check where the issuer verifies the billing address against the one on file, treats the result as a fraud signal rather than an automatic decline, and documents the issuer/country coverage limits explicitly.
• Note for review: the removed text also carried the "terminal prompts the operator when billing is omitted" behaviour, which is now documented nowhere. Say the word and I can add it back as a short note under the Billing section.

Test plan

• Visual review of the rendered restobjects.md page in the docs site

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