I had a lot of problems trying to download these docsets since updating to the latest Xcode. Various popups appearing saying it didn't know how to install it. Eventually after reading on here I just persisted and their servers started to work so i downloaded a few sets, iOS, WatchOS, and XCode so far. They're downloaded and show as having a tick box next to them.
However even though the docs are downloaded, when I go to use the documentation browser it requires an internet connection to show anything. Sometimes you see it rendering the webpage on apple's site to get the details and then it redraws in the usual documentation form.
I've a few docsets yet to download, but I'm not that bothered about them at this time.
Has anyone else encountered this and got any suggestions?