Where is this documented? I have been including FENs which have mostly turned into diagrams, but I suspect someone may be helping.
If I give an example of how to do it then you will just see the diagram!
The best way is for you to edit a post which has such a diagram so that you can see exactly how it is done. Copy the part that is displaying the diagram, paste it into your text editor, change it with the data for your game and then try posting it. Of course you should just cancel out of the edit of the original post.
That's how I did it the first time and even now if I want to include a diagram I edit one of my old posts to copy and paste a working example.