The only place that I can find polished documentation about stdlib, Foundation (corelibs and otherwise), is in the Apple documentation. Swift.org actually points me at the Apple Developer pages from their page. I understand the source code is documented but, it doesn't seem to have the polish that is shown in the Apple documentation, or in the Swift Language Reference e-book, and it's not organized except on a file-by-file basis. At least, I couldn't find anything like .html files, Markdown files or ReST files that contained the information that is in the Apple Developer pages outside of the comments in the source code files. And, the Apple documentation is included in the Xcode installation (albeit, hard to find). If you are looking for that level of documentation for off-line use, it seems like Apple is the only source right now, and its stance on re-distributing the Xcode documentation is uncertain at best.
And, if an organization were to develop that level of documentation based off the source code comments, augmented to the level desired, it's still not clear what it's re-distribution rights would be, as a total package.
Who should generate that that level of documentation and how it gets generated, if it does not exist outside of Apple, is a separate issue.