Add documentation.

docs/graph2d.html

@@ -238,6 +238,15 @@ groups.add({
       an individual css style.
     </td>
   </tr>
+  <tr>
+    <td>style</td>
+    <td>String</td>
+    <td>no</td>
+    <td>This field is optional. A style can be used to give groups
+      an individual css style, and any style tags specified in style will
+      override the definition in the className style defined in css.
+    </td>
+  </tr>
   <tr>
     <td>options</td>
     <td>JSON object</td>