documentation grdgradient -E (illumination of DEMs) needs clarification

[KristofKoch]

I had problems understanding that GMT's illumination of DEMs is not illumination in the traditional sense. It doesn't include cast shadows. I ran into that problem in October 2020 and John had the same problems this month.

While the grdgradient -E workings are briefly described in the technical reference 14.5 Artificial Illumination I find this quite lacking and therefore propose to explain in greater detail what exactly can be expected when using grdgradient -E.