doc: add description how BackgroundCleanupJob.getNewPreviewLocations works

[kesselb]

• Resolves: /

Summary

To understand the find previews to delete logic you need some background information how previews work. This description should help.

TODO

• [x] Todo

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [x] Tests (unit, integration, api and/or acceptance) are included
• [x] Screenshots before/after for front-end changes
• [x] Documentation (manuals or wiki) has been updated or is not required
• [x] Backports requested where applicable (ex: critical bugfixes)