auth.conf is there for backwards compatibility.
Don't use it if you're working from scratch. Instead use the server//conf/login-config.xml
Read the AdminDevel Docs section "An XML JAAS Login Configuration MBean" to understand how to edit the file.
Further down, in "Using and Writing JBossSX Login Modules" you'll find the IdentityLoginModule section covers the similarities between the old files and the new xml config way.