@boky Congrats. You've found an issue. The documentation really shouldn't be telling you to just run --help to get documentation. The valid arguments should be documented there instead.
Also, webapp-container should probably do argument parsing before trying to connect to the display. Though, this may be an issue with how Qt works in this case, and more difficult to fix.
You can report issues at https://github.com/ubports/morph-browser for these.