Update MIGRATION document

[emlaver]

Checklist

• [x] Tick to sign-off your agreement to the Developer Certificate of Origin (DCO) 1.1
• [x] migration doc change
• [x] Updated the change log file (CHANGES.md|CHANGELOG.md) or test/build only changes
• [x] Completed the PR template below:

Description

Clarify that db.list using design document is equivalent to the generated SDKs postDesignDocs in the migration document reference table.

Approach

Schema & API Changes

Security and Privacy

Testing

Monitoring and Logging