add legend format tip

public/app/plugins/datasource/cloudwatch/partials/query.editor.html

@@ -230,6 +230,7 @@
                  ng-model-onblur
                  ng-change="refreshMetricData()"
                  >
+          <tip>Syntax: {{Region}} {{Namespace}} {{MetricName}} {{Statistics}} {{Dimensions[N].Name}} {{Dimensions[N].Value}}</tip>
         </li>
       </ul>