The official documentation is at: http://docs.alfresco.com
Just a thought on wiki since someone put this here: A wiki based on Alfresco is in the position to support a concept which one might call 'federated wiki'. I am an active member of several open source communities and I like to form cross-over where I can. Some times there are wiki pages which I would like to maintain on both projects. They are the same wiki page. Let me give an example: Alfresco and Liferay enterprise portal. I would like our integration pages to be shared. If we were both running Alfresco wiki we could federate these pages -- keeping them in synch.
This concept helps branch communities and improve documentation.
Alfresco is already does and will likely continue to grow the number of integrated components. I would love to see federated documentation where it makes sense.