Better docs for levels.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1519744 13f79535-47bb-0310-9956-ffa450edef68
Better docs for levels.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1519745 13f79535-47bb-0310-9956-ffa450edef68