docs: added documentation and cli parsing for --class option

Update man pages and cli parsing output for the --class option, used to configure the WM class of the terminal. This is useful if you want to spawn terminals which will not stack together on the panel.

This functionality is already in xfce4-terminal, I have just documented it.

Refer to this forum post for more info: https://forum.xfce.org/viewtopic.php?pid=76645#p76645