We must edit maven-xdoc.dtd in the ${plugin.resources}/dtd directory
but never maven-xdoc-${currentRelease}.dtd in src/dtd.

Every time a new version of the plugin is released, we copy
${plugin.resources}/dtd/maven-xdoc.dtd into
src/dtd/maven-xdoc-${currentRelease}.dtd.

We store maven-xdoc-${currentRelease}.dtd in the SCM to be able
to re-publish them if necessary.