docs(specification.md): clarify AP2 specification doc

[baembry-goog]

Description

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Update AP2 specification with clarifications and flow diagram step corrections.

Based on partner feedback, this CL refines the definitions of Credentials Provider and Merchant Endpoint, adds details on merchant-forced user confirmation, and clarifies the accountability shift with a Cart Mandate. It also corrects several step numbers in the flow diagram explanation.

[gemini-code-assist]

Summary of Changes

Hello @baembry-goog, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the AP2 specification document by incorporating crucial clarifications and corrections based on partner feedback. The changes aim to enhance the precision of key definitions, elaborate on merchant-forced user confirmation processes, and adjust accountability with the introduction of a Cart Mandate. Additionally, it rectifies inconsistencies in the flow diagram's step numbering, ensuring the document is more accurate and easier to understand.

Highlights

• Clarified Definitions: The definitions for 'User's Credentials Provider (CP)' and 'Remote or Merchant Endpoint (ME)' have been expanded to provide more precise descriptions of their roles and scope within the AP2 ecosystem.
• Merchant-Forced User Confirmation: Detailed explanations have been added regarding scenarios where a merchant can force user confirmation, particularly when the request is not for a specific SKU, and how this mechanism impacts accountability.
• Cart Mandate Accountability Shift: The documentation now explicitly states that a 'Cart Mandate' shifts accountability for a bad transaction towards the user, except in cases of clear merchant error.
• Flow Diagram Step Corrections: Several step numbers referenced in the explanation of the flow diagram have been corrected to align accurately with the diagram's sequence.

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[gemini-code-assist]

Code Review

This pull request provides several useful clarifications to the AP2 specification, particularly around the definitions of ecosystem roles, merchant-forced confirmation, and accountability shifts. The corrections to the flow diagram step numbers are also helpful. I've found a minor formatting issue in the list of salient points for the flow diagram where a line of text was misplaced during the renumbering. My suggestions aim to correct this formatting to improve readability. Overall, these are great improvements to the documentation.