Clarify messaging when key backup exists but is unused

This makes the key backup panel much more explicit about the case where an
existing backup does exist, but this device just isn't using it. You can join
the device to the backup and restore from it by going through the restore
workflow.

Fixes https://github.com/vector-im/riot-web/issues/9446

src/components/views/settings/KeyBackupPanel.js

@@ -187,10 +187,16 @@ export default class KeyBackupPanel extends React.PureComponent {
                 clientBackupStatus = <div>
                     <p>{encryptedMessageAreEncrypted}</p>
                     <p>{_t(
-                        "This device is <b>not backing up your keys</b>.", {},
+                        "This device is <b>not backing up your keys</b>, " +
+                        "but there is an existing backup you can restore from " +
+                        "and add to going forward.", {},
                         {b: sub => <b>{sub}</b>},
                     )}</p>
-                    <p>{_t("Back up your keys before signing out to avoid losing them.")}</p>
+                    <p>{_t(
+                        "Enable key backup on this device via the restore " +
+                        "process before signing out to avoid losing any keys " +
+                        "that may only be on this device.",
+                    )}</p>
                 </div>;
             }