explain that transfinite surfaces with 3 or 4 corners can be specified automatically

tutorial/t6.geo

@@ -53,3 +53,20 @@ Recombine Surface{1};
 Mesh.Smoothing = 100;
 
 Physical Surface(1) = 1;
+
+// When the surface has only 3 or 4 control points, the transfinite constraint
+// can be applied automatically (without specifying the corners explictly).
+
+Point(7) = {0.2, 0.2, 0, 1.0};
+Point(8) = {0.2, 0.1, 0, 1.0};
+Point(9) = {-0, 0.3, 0, 1.0};
+Point(10) = {0.25, 0.2, 0, 1.0};
+Point(11) = {0.3, 0.1, 0, 1.0};
+Line(10) = {8, 11};
+Line(11) = {11, 10};
+Line(12) = {10, 7};
+Line(13) = {7, 8};
+Line Loop(14) = {13, 10, 11, 12};
+Plane Surface(15) = {14};
+Transfinite Line {10:13} = 10;
+Transfinite Surface{15};