The only piece of user documentation is the user guide here. Which you can also reach from Help > User guide menu in Spark.
When you run Spark for the first time it shows the login screen, where you can put your existing account credentials. If you go to Accounts menu, there you can create new accounts (this is optional).
Spark indeed uses Java, but it uses rather old Swing technology and is almost abandoned currently (no developers, only occasionally contributors). You can check what other XMPP clients are on Linux https://xmpp.org/software/clients.html
Btw, Spark officially supports only Java 8. It runs on newer Java, but some components might not work (plugins) and there can be some weird behavior. Maybe new account screen popping up was one of them.