If you add the following to you project.xml file, it will include the
required deps to run this goal- at least it did for me :)

<dependency>
	<groupId>xdoclet</groupId>
	<artifactId>xjavadoc</artifactId>
	<version>1.0.2</version>
	<url>http://xdoclet.sourceforge.net</url>
</dependency>
<dependency> 
	<groupId>xdoclet</groupId> 
	<artifactId>xdoclet-hibernate-module</artifactId> 
	<version>1.2</version> 
</dependency> 	
<dependency> 
	<groupId>xdoclet</groupId> 
	<artifactId>maven-xdoclet-plugin</artifactId> 
	<version>1.2</version> 
	<type>plugin</type> 
</dependency>