Being able to change the API version you're looking at while looking at the docs would be great.
It would be similar to the MSDN. See image.
Since this is in the core docs issue tracker, I'll ask: Is this something we'd want for core? I don't think our version diffs are big enough to bother.
Well, core can't do this since you don't have versions split out. I recently added a list of versions in the UI sidebar, but it doesn't retain state (so if you're looking at 1.10 dialog and click 1.9, you'll be on the 1.9 homepage, not the 1.9 dialog page).