Better docs.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1512787 13f79535-47bb-0310-9956-ffa450edef68
Add missing thresholdFilter element. (XMLSpy looks like it formated things around a bit though.)

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1512792 13f79535-47bb-0310-9956-ffa450edef68