I've never worked with any 3D and the only experience I have is looking at the formats.
You'll probably have a better idea what the data would represent. That would be useful because if I were given a set of values stored in a particular location (eg: when defining the mesh), I wouldn't know what it could be.
So a glossary would be useful.
The guide would be the tutorial-like page, while the glossary is a list of stuff that engines might use.
The guide itself will be fairly simplistic. I mean, it's not that much different from trying to reverse any other format, and you can only go so far. An explanation of how all the stuff works would eventually take up the majority of the page.
The glossary will be the main focus, cause I can quickly identify a struct, but have no clue what the struct holds.
You can handle the structure of the glossary. Maybe you might group it by concept (mesh, material, animation, etc) or alphabetically (though that might not be as useful).
Some things that I would find useful:
Simple transformations like rotating, scaling, and transforming. How the vectors look and possibly why that is the case (though that is more mathematical so maybe it's not too important)
Bones and stuff. Never played around with 3D editor so never made a model. Not really sure how stuff ties in with one another.
Animation. There are keyframes I guess? That's all I know lol