ndnsec: add a short description to the `--help` output of each command

Change-Id: Ifcc217bb096f1f27bceb609f5f117cf8b1d0c73d
diff --git a/tools/ndnsec/export.cpp b/tools/ndnsec/export.cpp
index 5641e08..0350bac 100644
--- a/tools/ndnsec/export.cpp
+++ b/tools/ndnsec/export.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2023 Regents of the University of California.
+ * Copyright (c) 2013-2024 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -47,6 +47,8 @@
   po::options_description visibleOptDesc(
     "Usage: ndnsec export [-h] [-o FILE] [-P PASSPHRASE] [-i|-k|-c] NAME\n"
     "\n"
+    "Export a private key and its certificate in SafeBag format.\n"
+    "\n"
     "Options");
   visibleOptDesc.add_options()
     ("help,h", "produce help message")