Allows API clients to update articles, which *content* has been modified since the last sync https://github.com/jocmp/capyreader/discussions/533#discussioncomment-11341808 https://github.com/FreshRSS/FreshRSS/issues/2566 Related: * https://freshrss.github.io/FreshRSS/en/developers/06_GoogleReader_API.html * https://www.inoreader.com/developers/item-ids * https://feedhq.readthedocs.io/en/latest/api/reference.html#stream-contents