docs(spec): clarify HTTP payment required header

[EfeDurmaz16]

Summary

• Cross-reference the HTTP transport from the core v2 PaymentRequired schema section.
• Clarify that HTTP carries the canonical PaymentRequired object in the base64 PAYMENT-REQUIRED response header.
• Note that response bodies may duplicate this information, but protocol clients/crawlers should not depend on body-only data.

Context

• Addresses the documentation ambiguity discussed in CDP Facilitator /verify and /settle do not return EXTENSION-RESPONSES header (v2 bazaar extensions in paymentPayload) #2207 around implementations returning PaymentRequired only in the response body.

Verification

• git diff --check

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