Upgrade usage.htm to make it clear how configuration is done

See http://stackoverflow.com/questions/8975096/maven-checkstyle-configlocation-ignored

There was some confusion on why the configuration was not being applied

http://maven.apache.org/plugins/maven-checkstyle-plugin/usage.html merges the tasks of "Generate Checkstyle Report As Standalone" and "Configure the Checkstyle Plugin" into one section.

Suggest creating a separate suggestion and link to Maven 3 Compatibility Notes, and indicate for Maven 2 that duplication may be required between build and reporting sections.