Allows API clients to update articles, which content has been modified since the last sync
jocmp/capyreader#533 (reply in thread)
#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