LOG4J2-710 Added documentation for Custom Levels and Custom Logger Wrappers

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1611386 13f79535-47bb-0310-9956-ffa450edef68
LOG4J2-710 (docs for Custom Levels/Custom Loggers) forgot to include link in user-guide pdf, small fix in manual page

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1611390 13f79535-47bb-0310-9956-ffa450edef68