Update documentation for windup-maven-plugin

Because WINDUP-1986 is complete, the documentation needs to be updated in line with the changes for that ticket.

As of now, you do not need to have the windupVersion parameter populated in the pom.xml when you're running the plugin. If this parameter isn't present, the plugin will default to its own build version. If the parameter is present, its value will override the default and the plugin will use that version of windup.