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

Change-Id: Ifcc217bb096f1f27bceb609f5f117cf8b1d0c73d
diff --git a/tools/ndnsec/get-default.cpp b/tools/ndnsec/get-default.cpp
index 8dba1bf..3d3a7f2 100644
--- a/tools/ndnsec/get-default.cpp
+++ b/tools/ndnsec/get-default.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).
  *
@@ -38,6 +38,8 @@
   po::options_description description(
     "Usage: ndnsec get-default [-h] [-k|-c] [-i ID|-K KEY] [-q]\n"
     "\n"
+    "Show the default identity, key, or certificate.\n"
+    "\n"
     "Options");
   description.add_options()
     ("help,h", "produce help message")