add simple doc

src/experimental/tf_torus.scad

@@ -1,5 +1,12 @@
 use <rotate_p.scad>;
 
+/*
+    size: The size of a rectangle.
+    point: A point in the rectangle.
+    radius: torus [R, r]
+    angle: torus [A, a].
+    twist: The number of degrees of through which the rectangle is twisted.
+*/
 function tf_torus(size, point, radius, angle = [360, 360], twist = 0) =
     let(
         xlen = size[0],