What a mess. A standard library and then apparently on top of that a Core and that's where they put a Foundation, you can imagine Apple architects arguing with the elevator contractors, "No, no, the Ground floor of our building is below our Foundation, it's very straight forward..."

But although that's enough for me to want no part of it, that's not what I was gesturing at. When we dig under the "Foundation" to look at the standard library we find that contrary to your assurance what works and how it works varies from one platform to another, just Apple only care about Apple platforms and so as usual they don't think that's worth mentioning.

They do have this information, they just don't publish it on their Apple pages because they're Apple and yet they insist this counts as the Swift documentation - and that should be all the reason you need not to take such "support" seriously.