Javadoc addition.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1617169 13f79535-47bb-0310-9956-ffa450edef68
Clarify error output for running PluginManager.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1617170 13f79535-47bb-0310-9956-ffa450edef68
Add and use method to add multiple packages to PluginManager.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1617171 13f79535-47bb-0310-9956-ffa450edef68