Document the differences of know 2D and 3D variables in custom materials

When creating 2D or 3D shaders for custom materials, user must create two different instances of the files to be able to make 2D or 3D material which is OK but we don't have documentation that outlines what is accepted as variables in 2D and 3D. 

Requirement is to get a document (or wiki page) where the known variables are told per dimension type and possible migration guide.