I tend to explain what it is, and what is the important part. Thinking about it from the perspective of what someone might need to know while also respecting their time. I think “Screen shot of Lemmy post feed with nav bar at top, first post says “blah”, second post says “blah blah”, third post says…”, I think that’s too much unnecessary detail. So I’d do something like “Screen shot of Lemmy post feed, showing the third post called “Blah blah” has a green highlight over the first word” or whatever the message is I’m trying to get across with the screenshot.

I don’t know if there is etiquette or a specific format but I would write as much as is needed to convey the reason you’re including the image (whether that is a sentence or 100 words), striking a balance between making sure someone who is relying on the alt text can understand everything they need to know while also respecting their time.