Unfortunately Microsoft puts them behind a developer account and NDA requirement. The less specific docs can be viewed here but they tend to only make vague references to the restrictions (mentioning things like API service needing to be discussed and set up with your point of contact, etc.): https://learn.microsoft.com/en-us/gaming/gdk/_content/gc/live/features/multiplayer/live-multiplayer-intro

The problem with cross console on private servers tends to come from the rules set forth by the company. Microsoft has a lot of restrictions on how cross platform multi-player can be implemented, and since the servers are being hosted by the players and not the company they most likely aren’t able to meet the requirements.