That's the old documentation style. They redesigned their documentation last year. It hasn't made it to all documentation yet, but new stuff uses the new design. Still has some strange UI bugs, but it's at least responsive now.
I assume it's the same effect I see with Opera (blink) on Android. Fixed positioning and small screens don't play well together, especially when zooming. They should have designed it with also a mobile layout but probably they didn't think it was worth the trouble for that kind of site. I expect that most of its audience comes from Macs, so at least 11" screens.
I'm trying to think of a time I would ever need to access technical documentation on a phone. Developers typically access documentation when developing and they typically develop on large (over 13") screens.
https://developer.apple.com/library/ios/documentation/Swift/...