docs: clarify saved fulfillment destination selection

[karangoel16]

Summary

• Document in Identity Linking that businesses may return saved fulfillment destinations for identity-linked sessions, and platforms select them through existing id-bearing fulfillment surfaces.
• Clarify that identity-linked buyer email/phone/name values remain connected to the authenticated user but are submitted as scalar values unless an id-bearing extension or business-specific field exists.
• Add concise REST and MCP checkout-update cross-references to the reusable Identity Linking guidance instead of duplicating request examples.

Validation

• git diff --check
• uv run mkdocs build

Notes: MkDocs build completed with existing local cairosvg/Cairo warnings.