document spline interpolation type options (#2513)

Options for the type of spline interpolation methods aren't documented -- or at least I couldn't find it in docs. Option was added with #1268

htdocs/samples/chart_spline.html

@@ -18,7 +18,12 @@
             data1: 'spline',
             data2: 'spline'
           }
-        }
+        },
+        spline: {
+          interpolation: {
+            type: 'monotone', // 'cardinal' is default
+          },
+        },
       });
     </script>
   </body>