Code assist was improved in JBT 3.1 (See https://community.jboss.org/wiki/NewKBplug-inandCodeAssistinJBossTools310M3 for details). And now we pick up info about components from TLDs/facelet libs that are available for the project. If you don't have corresponding libs in the classpath (and don't want to add them to it) then you can add them to XML Catalog via preferences page. See https://community.jboss.org/wiki/NewKBplug-inandCodeAssistinJBossTools310M3
In JBT 3.0 this meta info was hardcoded.