Harmonize wxGraphicsPathData::AddArcToPoint() behaviour across all ports

AddArcToPoint() on macOS port is implemented with native API (CGPathAddArcToPoint) so its behaviour should be considered as a reference for generic implementation used in another ports.

Closes #18086.

interface/wx/graphics.h

@@ -50,9 +50,14 @@ public:
     //@}
 
     /**
-        Appends a an arc to two tangents connecting (current) to (@a x1,@a y1)
-        and (@a x1,@a y1) to (@a x2,@a y2), also a straight line from (current)
-        to (@a x1,@a y1).
+        Adds an arc (of a circle with radius @a r) that is tangent
+        to the line connecting current point and (@a x1, @a y1) and
+        to the line connecting (@a x1, @a y1) and (@a x2, @a y2).
+        If the current point and the starting point of the arc are different,
+        a straight line connecting these points is also appended.
+        After this call the current point will be at the ending point
+        of the arc.
+        @image html drawing-addarctopoint.png
     */
     virtual void AddArcToPoint(wxDouble x1, wxDouble y1, wxDouble x2,
                                wxDouble y2, wxDouble r);