Updated plugin tests to verify we can generate the dashboard report by using the multiproject:site goal. Also updated documentation accordingly.

git-svn-id: https://svn.apache.org/repos/asf/maven/maven-1/plugins/trunk@115094 13f79535-47bb-0310-9956-ffa450edef68

dashboard/src/plugin-test/subproject2/project.xml

@@ -22,6 +22,8 @@
   <id>test-maven-dashboard-plugin-subproject2</id>
   <name>Maven Dashboard plugin test - Subproject2</name>
   <reports>
-    <report>maven-checkstyle-plugin</report>
+    <!-- Voluntarily do not put any report. We're forcing the execution
+         of artifact goals using the maven.dashboard.rungoals=true 
+         property -->
   </reports>
 </project>