[calamares] Document how to hide the Quit button

The Quit button can have its own logic at a QML level for
show/hide. It **ought** to follow the *quitVisible* property,
but can do additional work. Here, document how a distro might
choose to hide the Quit button on the last page (generally,
that's the "finished" page).

src/calamares/calamares-navigation.qml

@@ -46,10 +46,13 @@ Rectangle {
             enabled: ViewManager.nextEnabled;
             visible: ViewManager.backAndNextVisible;
             onClicked: { ViewManager.next(); }
+            // This margin goes in the "next" button, because the "quit"
+            // button can vanish and we want to keep the margin to
+            // the next-thing-in-the-navigation-panel around.
+            Layout.rightMargin: 3 * buttonBar.spacing;
         }
         Button
         {
-            Layout.leftMargin: 3 * buttonBar.spacing;  // little gap from back/next
             Layout.rightMargin: 2 * buttonBar.spacing
             text: ViewManager.quitLabel;
             icon.name: ViewManager.quitIcon;
@@ -59,6 +62,19 @@ Rectangle {
             ToolTip.delay: 1000
             ToolTip.text: ViewManager.quitTooltip;
 
+            /*
+             * The ViewManager has settings -- user-controlled via the
+             * branding component, and party based on program state --
+             * whether the quit button should be enabled and visible.
+             *
+             * QML navigation *should* follow this pattern, but can also
+             * add other qualifications. For instance, you may have a
+             * "finished" module that handles quit in its own way, and
+             * want to hide the quit button then. The ViewManager has a
+             * current step and a total count, so compare them:
+             *
+             * visible: ViewManager.quitVisible && ( ViewManager.currentStepIndex < ViewManager.rowCount()-1);
+             */
             enabled: ViewManager.quitEnabled;
             visible: ViewManager.quitVisible;
             onClicked: { ViewManager.quit(); }