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Angular probably uses typescript-documentation , which means what you see on angular.io docs is written and sent to github with actual code in the same typescript file.
angular.io docs have a github link for this file in bottom-right . And since we all know how github works, you can just look at the history of a specific document , however there is no clear markup by version, just do commit seperation .
Update: You can also select tag (version) from the top left in the github file shown in the second snapshot. - @Scrambo (comment)
Here is a stream with screenshots,
