The documentation you have found is just for the Xamarin.iOS wrappers on top of our UI for iOS suite that are included in UI for Xamarin. If you are interested in the controls that work for Xamarin.Forms, please refer to the following documentation: UI for Xamarin Documentation. Calendar for Xamarin.Forms is not supported yet though. It will be in the upcoming release in mid-December.
Generally speaking, all documentation sets are linked from the UI for Xamarin product page, and more specifically from the Support and Learning section.
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