You can configure any custom tags used in your javadoc via the <tags/> element:
<project>
...
<reporting>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-javadoc-plugin</artifactId>
<configuration>
...
<tags>
<tag>
<name>todo</name>
<!-- Should be a combination of the letters Xaoptcmf -->
<placement>a</placement>
<head>To do something:</head>
</tag>
</tags>
...
</configuration>
</plugin>
</plugins>
...
</reporting>
...
</project>