Update usage doc

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@31856 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/tests/runFunctionalTest.py

@@ -32,6 +32,7 @@ Options:
                                  http://${host}:${port}/${portal_name}/
   --run_only=STRING          run only specified test suite (should be only one)
   --email_subject            the email subject to be sent
+  --xvfb_display=STRING      Define a xvfb display to be used.
 Notes:
   * You need to prepepare first test environment by using following command:
   ./runUnitTest.py --save prepareFunctionalTest.py