The docs are part of the "Developer's guide":
http://docs.jboss.org/hibernate/orm/4.2/devguide/en-US/html/ch15.html

I'm not quite sure what's the distinction between the dev guide and the reference docs

There's also the very old page when Envers was not part of Hibernate: http://envers.jboss.org

Maybe this could get migrated to hibernate.org - but I'm not sure who would be the person to talk to about that.