This part does not exist yet. We have hired a full group of meticulous code monkeys just to write the documentation here. Sadly they are on a strike for more bananas.
In the meantime we will do our best to add this content our self.
Afx is a kind of preprocessor that converts XML syntax to Fusion. This way you can basically write normal HTML while having access to Neos Variables, Components, Eel Helper and of cause Fusion.
What's cool about AFX? > Native Fusion Code
Talk about syntax in isolation from other topics. (fusion, atomic fusion, components, nodetypes)
How to access data given by Fusion?
--> Don't talk about components and presentational. AFX can work without it and it's easy to be confused by too many new concepts.