LOG4J2-759 minor site/documentation issues; mostly changing mentions of "Log4j 2.0" to "Log4j 2" to prepare for upcoming 2.1, 2.2 etc releases; also update for custom log levels

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1615406 13f79535-47bb-0310-9956-ffa450edef68