Add explanation about Normal+Y / Normal-Y

[struffel]

The material definition in AF has a way for declaring normal maps to be either in OpenGL or DirectX "notation".

This needs a bit more explanation to be understandable.

[audiotrope]

How much background knowledge do we assume the reader of the documentation has?

[struffel]

I think it is safe to assume that the reader knows what a PBR material and what a normal map is.
But the normal+y and normal-y fields in 7.74 are a bit confusing on their own.