rework specular description for PRM

[ScanMountGoat]

This should just describe how specular is converted to IOR and give a few sample images of the fresnel curve and resulting rendered image for an in game model. The existing explanation is a bit technical and requires too much intuitions about surface reflectance.

• rework specular description
• show a comparison with mario for fresnel and rendered result for different specular values
• show an example nodegroup in Blender

[ScanMountGoat]

It might be clearer to just show what a fresnel curve looks like and only give a brief explanation. The technical details aren't relevant and require too much notation to express clearly.