Updated the xjc.html page (https://jaxb.dev.java.net/nonav/2.2.1/docs/xjc.html)
to provide a list of the Add-on extensions in the "Output" section, also
provided explanations of the inject-code and propertyaccessors plugin.

Updated the PluginImpl.java class to provide a clearer definition of the
plugin's purpose for newbies.

Environment

Operating System: All
Platform: All

Affected Versions

[2.3]