Document alternative Synaptic dialog with software-properties-gtk
Someone reported that the installation instructions on /install/usb/debian are incorrect if software-properties-gtk is installed. A different dialog is displayed in Settings ▸ Repositories that has no “New” button.
Note that this package is not installed by default on Debian Jessie.
Some more info was initially posted on #12387 (closed).
Feature Branch: doc/11110-software-properties-gtk