Use the following command to generate asdoc:
$ mvn org.sonatype.flexmojos:flexmojos-maven-plugin:asdoc
It is possible to add it to default building process adding an extra execution
<build>
<plugins>
<plugin>
<groupId>org.sonatype.flexmojos</groupId>
<artifactId>flexmojos-maven-plugin</artifactId>
<executions>
<execution>
<phase>install</phase>
<goals>
<goal>asdoc</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>