[TD] comment the new centerline behavior

add missing info to understand the new centerline behavior as introduced with commit d1476c64

src/Mod/TechDraw/App/Cosmetic.cpp

@@ -980,12 +980,16 @@ std::pair<Base::Vector3d, Base::Vector3d> CenterLine::calcEndPoints(DrawViewPart
 
 bool CenterLine::Circulation(Base::Vector3d A, Base::Vector3d B, Base::Vector3d C)
 {
+    // the determinant of this matrix calculates the aread of a triangle, see
+    // https://en.wikipedia.org/wiki/Triangle#Using_coordinates
+    // a 3x3 matrix would also do the job, but FC supports only 4x4 matrixes
     Base::Matrix4D CircMatrix(
         A.x, A.y, 1, 0,
         B.x, B.y, 1, 0,
         C.x, C.y, 1, 0,
         0, 0, 0, 1);
 
+    // the sign delivers the dicrection of travel along the triangle edges
     if (CircMatrix.determinant() > 0)
         return true;
     else
@@ -1034,8 +1038,10 @@ std::pair<Base::Vector3d, Base::Vector3d> CenterLine::calcEndPoints2Lines(DrawVi
     Base::Vector3d l2p2 = edges.back()->getEndPoint();
 
     // The centerline is drawn using the midpoints of the two lines that connect l1p1-l2p1 and l1p2-l2p2.
-    // However, we don't know which point should be l1p1 to get a geometrically correct result.
-    // Thus we test this by a circulation test.
+    // However, we don't know which point should be l1p1 to get a geometrically correct result, see
+    // https://wiki.freecadweb.org/File:TD-CenterLineFlip.png for an illustration of the problem.
+    // Thus we test this by a circulation test, see this post for a brief explanation:
+    // https://forum.freecadweb.org/viewtopic.php?p=505733#p505615
     if (Circulation(l1p1, l1p2, l2p1) != Circulation(l1p2, l2p2, l2p1)) {
         Base::Vector3d temp; // reverse line 1
         temp = l1p1;