Material Expressions Documentation in Source

Hi! I’m looking for information on how to create Material Expressions for the Material Editor and I actually found enough information to get me around for the time being, but I was curious where the documentation mentioned in this post (https://forums.unrealengine.com/t/creating-own-material-nodes/366080/2) is, since I downloaded the Unreal Engine source and didn’t find any docs in the path:

UnrealEngine/Engine/Documentation/Source/Programming/Rendering/MaterialExpressions/

In fact, there was nothing under […]/Programming/Rendering

Does anyone know where can I find these documents?