Clarify that sorting rooms by activity isn't the only thing it does

Recently, it also sorts rooms based on unread notification count and
importance. This adds a clarification to the setting so users (like me)
aren't confused why it isn't sorting only by activity.

src/settings/NeoChatGeneralPage.qml

@@ -93,6 +93,7 @@ FormCard.FormCardPage {
     FormCard.FormCard {
         FormCard.FormRadioDelegate {
             text: i18nc("As in 'sort something based on last activity'", "Activity")
+            description: i18nc("@info", "Rooms with unread notifications will be shown first")
             checked: NeoChatConfig.sortOrder === 1
             enabled: !NeoChatConfig.isSortOrderImmutable
             onToggled: {