add simple doc

src/experimental/pt_to_sphere.scad

@@ -1,6 +1,12 @@
 use <rotate_p.scad>;
 
-function pt_to_sphere(p, size, r, angle = [180, 360]) =
+/*
+    size: The size of the rectangle mapping to a sphere.
+    p: A point in the rectangle.
+    r: sphere radius.
+    angle: [za, xa] mapping angles.
+*/
+function pt_to_sphere(size, p, r, angle = [180, 360]) =
     let(
         x = p[0],
         y = p[1],