Document alternative Synaptic dialog with software-properties-gtk

Originally created by @sajolida on #11110 (Redmine)

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

Related issues

• Related to #11706 (closed)
• Related to #12372 (closed)
• Has duplicate #12387 (closed)