Update wxDC::DrawSpline() documentation

Mention that drawn spline is not an interpolating curve.

interface/wx/dc.h

@@ -716,9 +716,14 @@ public:
 
         The number of points must be at least 2 for the spline to be drawn.
 
+        @note Drawn curve is not an interpolating curve - it does not go
+        through all points. It may be considered a smoothing curve.
+
         @beginWxPerlOnly
         Not supported by wxPerl.
         @endWxPerlOnly
+
+        @image html drawing-spline.png
     */
     void DrawSpline(int n, const wxPoint points[]);