Document user request for arb validator

Probably makes the most sense for lint and/or as a subcommand of an arb
primitive command - need something to audit existing arbs looking for
problematic definitions.

Author: starseeker

TODO

@@ -51,6 +51,9 @@ THESE TASKS SHOULD HAPPEN WITHIN TWO RELEASE ITERATIONS
 THESE ARE UNSCHEDULED BACKLOG TASKS
 -----------------------------------
 
+* user request for a way to scan arbs in a .g looking for invalid
+  definitions (non-convex, non-planar faces, etc.)
+
 * implement nirt flag to report a secondary ray fired in the direction
   of the hit point surface normal.