Refer to global notification settings as "Default Settings" instead

This changes references to "global notification settings" to "default
settings", which should hopefully make it clearer what these actually
are.

Also, a information tip has been added to the settings page to clarify
what these settings do and imply that the per-room settings take
precedence.

src/rooms/RoomContextMenu.qml

@@ -58,7 +58,7 @@ KirigamiComponents.ConvergentContextMenu {
         icon.name: "notifications"
 
         Kirigami.Action {
-            text: i18n("Follow Global Setting")
+            text: i18nc("@action:inmenu Notification 'Default Settings'", "Default Settings")
             icon.name: "globe"
             checkable: true
             autoExclusive: true