REST API and template reference documentation for Management Groups is missing

This issue was created by @jtuliani from https://github.com/MicrosoftDocs/feedback/issues/577

IMPORTANT: You can get your feedback addressed faster if you use the comment section for the article in which you encountered a problem.
Understood. This issue relates to the Azure REST API and Azure template references, which don't (yet) have a comments section.
Link to article:
https://docs.microsoft.com/rest/api/?view=Azure
https://docs.microsoft.com/azure/templates
Problem:
Azure Management Groups are now GA: https://azure.microsoft.com/en-us/blog/azure-management-groups-now-in-general-availability/
However, I can't find any REST API or template reference documentation for Management Groups.