Updating the Prim Ref page

[Willy Tschager]

• I added the "Disc negative truncated" and would appreciate getting a meaningful description:
  
  "This suite of primitives are used to ..."
  
  The related discussion can be found here:
  
  https://forums.ldraw.org/thread-24731.ht...ight=tndis
  

• I also added the Nomenclature to the:
  
  https://wiki.ldraw.org/wiki/Primitives_R...primitives
  
  
• I will add the helix prims at a later point.
  

Cross-check before I move on to copy more content from the old side.

w.