I used the following "reportSets" and called "mvn clean site" and it works
for me.
<reporting>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-javadoc-plugin</artifactId>
<version>2.7</version>
<reportSets>
<reportSet>
<id>internal</id>
<reports>
<report>javadoc</report>
</reports>
<configuration>
<reportOutputDirectory>${project.reporting.outputDirectory}/javadocs</reportOutputDirectory>
<destDir>internal</destDir>
<show>private</show>
<nohelp>true</nohelp>
</configuration>
</reportSet>
<reportSet>
<id>external</id>
<reports>
<report>javadoc</report>
</reports>
<configuration>
<reportOutputDirectory>${project.reporting.outputDirectory}/javadocs</reportOutputDirectory>
<destDir>external</destDir>
<show>public</show>
<nohelp>false</nohelp>
</configuration>
</reportSet>
</reportSets>
</plugin>
</plugins>
</reporting>