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

Change-Id: Ifcc217bb096f1f27bceb609f5f117cf8b1d0c73d
diff --git a/tools/ndnsec/cert-install.cpp b/tools/ndnsec/cert-install.cpp
index 345ee24..f7ce2f0 100644
--- a/tools/ndnsec/cert-install.cpp
+++ b/tools/ndnsec/cert-install.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).
  *
@@ -42,6 +42,8 @@
   po::options_description description(
     "Usage: ndnsec cert-install [-h] [-I|-K|-N] [-f] FILE\n"
     "\n"
+    "Import a certificate from a file.\n"
+    "\n"
     "Options");
   description.add_options()
     ("help,h", "produce help message")