We started adding an "added_in" entry, which tells us in which version a particular field was added, everytime we introduce a new field in a web API resource. It was pretty inconsistant, and some fields never had those entires added in the first place.

Now, all relevant fields in the web API resources have an "added_in" entry, and all releveant resources in the web API docs have a "versionadded" entry with the correct version.

Used git blame, git log, and git tag to find the commit that added each field.

Built the docs.