TechDraw - Add edge midpoint cosmetic vertex fix - added comments

src/Mod/TechDraw/App/Geometry.cpp

@@ -326,15 +326,24 @@ Base::Vector3d BaseGeom::getEndPoint()
 
 Base::Vector3d BaseGeom::getMidPoint()
 {
+    // Midpoint calculation - additional details here: https://forum.freecadweb.org/viewtopic.php?f=35&t=59582
+
     BRepAdaptor_Curve curve(occEdge);
+
+    // As default, set the midpoint curve parameter value by simply averaging start point and end point values
     double midParam = (curve.FirstParameter() + curve.LastParameter())/2.0;
 
+    // GCPnts_AbscissaPoint allows us to compute the parameter value depending on the distance along a curve.
+    // In this case we want the curve parameter value for the half of the whole curve length,
+    // thus GCPnts_AbscissaPoint::Length(curve)/2 is the distance to go from the start point.
     GCPnts_AbscissaPoint abscissa(Precision::Confusion(), curve, GCPnts_AbscissaPoint::Length(curve)/2.0,
                                   curve.FirstParameter());
     if (abscissa.IsDone()) {
+        // The computation was successful - otherwise keep the average, it is better than nothing
         midParam = abscissa.Parameter();
     }
 
+    // Now compute coordinates of the point on curve for curve parameter value equal to midParam
     BRepLProp_CLProps props(curve, midParam, 0, Precision::Confusion());
     const gp_Pnt &point = props.Value();