docs: clarify downstream adapter validation requirements

[zeitlinger]

Summary

• document that downstream MultiCollector adapters must expose registration metadata that matches the metric families they emit at scrape time
• make the newer suffix/collision validation model more explicit for adapter maintainers

Why

• the existing release notes already describe the suffix-handling and collision-model change
• what was missing was explicit guidance that downstream adapters also need their registration-time metadata to stay aligned with emitted MetricSnapshot families

Testing

• mise run lint