docs: Emphasize requirement to provide filenames as arguments

(related to #5430 (closed))

@matthiasc What do you think of something like this?

I've tweaked the docs, so probably the changes I made to main() are not needed or desired. But I did that first so I figured I'd keep them in the MR until you or someone else had a chance to review them.

Another option would be to keep everything as it is (revert changes in this MR), but puts() a message to the console if no arguments arguments are supplied.

A 4th option would be to print a message in the window if no arguments are supplied.

A 5th option, as you said, would be to implement a file chooser (which I'm not ready to do yet). A file chooser would probably be more suitable for a separate example anyway (since the goal is to keep the examples small.)