it looks like the source declaration doesn’t have any availability annotations at all, so i’m surprised the Apple docs are displaying platform availability in the first place.
this problem is very similar to API availability on linux? . there isn't a good solution because the required source level information simply doesn't exist. (because the needed annotations aren't mandatory.)
perhaps we could add an availability attribute to it in the next swift release, though that wouldn't help anyone browsing docs for 5.8/9…