Hi Thea,
Please understand that majority of Forge component was from Community Member, and as such it's at creator discretion whether he/she will made documentation.
I wouldn't said it was deal breaker.
This one thing should not over-shadowing the whole greatness of Outsystems Platform.
Time spent exploring and making logic to synchronize AD Users and Groups to Outsystems Users and Groups is very minuscule compared to what you will save for development of web/mobile apps.
BTW, are you using on-premise Outsystems Platform server?
You said your AD server was on-premise.
Your AD server should be reachable from the internet if you're using Outsystems Cloud Server.