Fix unacceptably light-hearted string in serious business mode

Summary: A serious business lost a bunch of serious business partners today because of this string, I assume.

Test Plan: Enabled serious mode, clicked button, was relieved to see no jokes.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6799

src/applications/notification/controller/PhabricatorNotificationClearController.php

@@ -24,8 +24,19 @@ final class PhabricatorNotificationClearController
     $dialog = new AphrontDialogView();
     $dialog->setUser($user);
     $dialog->setTitle('Really mark all notifications as read?');
+
+    $is_serious = PhabricatorEnv::getEnvConfig('phabricator.serious-business');
+    if ($is_serious) {
       $dialog->appendChild(
-      "You can't ignore your problems forever, you know.");
+        pht(
+          "All unread notifications will be marked as read. You can not ".
+          "undo this action."));
+    } else {
+      $dialog->appendChild(
+        pht(
+          "You can't ignore your problems forever, you know."));
+    }
+
     $dialog->addCancelButton('/notification/');
     $dialog->addSubmitButton('Mark All Read');