I don't think it belongs in the docs folder, but it would be good perhaps to have some space somewhere on swift.org itself for this information, and also for the list of platforms, owners and tiers, so that's something we're looking at — but we wanted to get this published sooner rather than later, so for now it's in the form of a vision document.
Ownership of the existing platforms is something we need to talk to people about; one of the reasons we didn't publish the platform/owner/tier list at the same time is that there was some debate about who owned what. But yes, in principle, different Linux distributions might have their own distribution owner (which may be the same person in some cases).
I think it's intentionally vague — the point here is to make sure that we've got an implementation of Swift and not some sub-language that can't cope with anything other than what's in the SDK, rather than to stipulate exactly what needs to build. It is specifically not a requirement that e.g. every package on the Swift Package Index should build without errors.
I think for now I'd rather leave things as open as possible (and lean on "if applicable" as well as the notion that platforms may not need to strictly comply with every requirement to be in a tier, as long as there's a good reason for the non-compliance). My feeling is that we may codify more things in future as we develop experience with the process.