Updating the Prim Ref page

[Willy Tschager]

some of the descriptions are now incorrect due to the new rendered pics, for example:

n-ftang.dat: "Note that the bounding circle in the image is for context only - only the grey segment is generated by the 1-4tang primitive."
(but the new pic has a dark grey ring and a black n-ftang)

The other images still show light grey faces, so maybe change the pic instead.

Corrected. Description for "Disc negative truncated" still needed. Anyone?

w.