local-tx-datasource is defined in docs/schema/jboss-ds_5_0.xsd. There is also one at docs/dtd/jboss-ds_5_0.dtd, but I think the XSD is used. (This is for JBoss AS 5.1.0, other versions will be different.)
The rest of your question I do not understand. But to define a datasource you need to create a *-ds.xml file and place it in the delpoy directory. You can see many examples for different databases at docs/examples/jca