The docs-old site should ideally display a banner on every page warning users that they're viewing outdated content. We talked about doing this before, and adding the banner on every book that's current version minus two, but that proved unfeasible (we'd have to publish everything from the newly outdated release again which might not even be possible due to bad git discipline). Now that we have all the old content isolated on docs-old, it would probably be much easier to just slap a quick script into every page displaying a warning. Of course, F25 content isn't actually outdated yet, but I wouldn't worry about that too much.