Clarify StoreKit purchase and reconciliation paths

[lynnswap]

Purpose

Clarify which purchase flows are owned by the app and how StoreTransactionKit handles completed transactions and subscription-status changes.

Changes

• Document app ownership of PurchaseIntent.intents, purchase initiation, and forwarding Product.PurchaseResult to TransactionStore.process(_:).
• Clarify that completed transactions exposed through Transaction.updates or Transaction.currentEntitlements enter the store without app forwarding.
• Document subscription-status changes as reconciliation triggers rather than a second entitlement source.
• Give concrete App Store examples for unrecognized same-group subscriptions and broaden the process(_:) symbol documentation.

Testing

• git diff --check
• xcrun swift-format lint --recursive --strict Sources Tests Tools/TestApp Package.swift
• swift test --build-system swiftbuild --no-parallel (135 tests)
• xcodebuild docbuild -scheme StoreTransactionKit -destination 'generic/platform=macOS' -derivedDataPath .build/docc-local OTHER_DOCC_FLAGS='--warnings-as-errors'
• Codex review: no findings