Add note on multiple patterns.

As recommended on this old mailing list thread http://jenkins.361315.n4.nabble.com/Multiple-test-reports-td380558.html . Which I found after Googling for a hint on specifying multiple Jenkins JUnit reports.

war/src/main/webapp/help/tasks/junit/report.html

@@ -9,7 +9,7 @@
   specify the path to JUnit XML files in the
   <a href="http://ant.apache.org/manual/Types/fileset.html">Ant glob syntax</a>,
   such as <tt>**/build/test-reports/*.xml</tt>. Be sure not to include any non-report
-  files into this pattern.
+  files into this pattern. You can specify multiple patterns of files separated by commas.
 
   <p>
   Once there are a few builds running with test results, you should start seeing