eDenon-2 nouveau is for Nvidia GPUs. The xrandr --prop is not just for listing connected ports but it also lists available options and how they are named. On my card for example the option is not called dithering but "dithering mode". The option names differ between GPUs. You might want to add the user has to put options that include spaces into quotation marks. Something which people who aren't used to the command line might not know.
Also the current Nvidia teradici section is for Nvidias proprietary closed source drivers. Nouveau is an open source driver.
The nvidia-xconfig method is only applied after the user logged in, started his own Xorg session, and on top of that has "nvidia-xconfig --load-config-only" in his autostart. If he looks at a login manager he might look at dithering because the config is not loaded yet. With the global xorg.conf file method the dithering might be deactivated for login screens, too.
In case the option name ever changes, go here: https://www.nvidia.com/object/unix.html, choose your driver, select "Additional Information", then README, then one of the appendixes. I think it is something like "Appendix D Xconfig options". Sometimes there are no such links or the content is missing.