See this blog post for a howto: JavaDoc Plugin For Component Reference Documentation. You need to add the tapestry config to the reporting plugin.
If this configuration is put into the reporting tag, it will only be used when generating reports via mvn site. Calling mvn javadoc:javadoc does not use the reporting tag. However, you can easily solve your problem by putting it into the build tag.
<project>
...
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-javadoc-plugin</artifactId>
<version>2.8</version>
<configuration>
...
<taglet>org.apache.tapestry5.javadoc.TapestryDocTaglet</taglet>
<tagletArtifact>
<groupId>org.apache.tapestry</groupId>
<artifactId>tapestry-javadoc</artifactId>
<version>5.3-rc-3</version>
</tagletArtifact>
...
</configuration>
</plugin>
</plugins>
...
</build>
...
</project>