The docsets I seek are Xcode 6.2 and iOS 8.2. The machine is running an old macOS and can't be upgraded. Every time I click on offered items under "Preferences->Downloads->Documentation" I see the progress bar showing my request is acknowledged but then it halts halfway with the alert Could not download and install Xcode 6.2. The package “DevToolsDocset.pkg” is untrusted. I tried setting the date several years back but got another more verbose and less helpful alert message.
"Untrusted package" alert (imgur)
So, do there exist direct download links for Xcode 6.2 and iOS 8.2 docsets? What's a workaround to get them?
The solution came by itself quite unexpectedly so I'm sharing my trial and error experience: