Hey, it seems that xfdesktop manpage is a bit outdated. First, it's from déc 2 2007, so it's definitely quite old (I guess that's from 4.4 or 4.2 maybe :), and it lacks for example the --quit arg.
Updated the man page with --arrange and --quit options. I'll try to remember to update it when things get added. Pushed to master: http://git.xfce.org/xfce/xfdesktop/commit/?id=8874700332228013925ac157a7b00b8121f86aa1