In the man page. This has been the entire contents of nvidia-settings-user-guide.txt shipped with the driver for a long time:
Originally Posted by nvidia-settings-user-guide.txt
The nvidia-settings user guide has been moved to the nvidia-settings(1) man
The one on the FTP server was way out of date.
BTW, you can make a nice formatted version of the guide (or any man page, for that matter) by running
zcat /usr/share/man/man1/nvidia-settings.1.gz | groff -man | ps2pdf - nvidia-settings.pdf