Unfortunately the JBoss 6 documentation is not complete.
But lot of things will be similar in JBoss 4/5.
you might find such guiding using google to search 'jboss clustering guide'
I don't think there is one, but you should be able to use the 5.1 clustering guide as majority of the stuff still holds:
Some additional docs for AS6 are here:
From a user's perspective, clustering in AS6 is similar as 5.1 (so those docs apply), with the following changes:
The most notable change is the replacement of JBoss Cache with Infinispan: