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

Change-Id: Ifcc217bb096f1f27bceb609f5f117cf8b1d0c73d
diff --git a/tools/ndnsec/key-gen.cpp b/tools/ndnsec/key-gen.cpp
index bd3712e..77c7d57 100644
--- a/tools/ndnsec/key-gen.cpp
+++ b/tools/ndnsec/key-gen.cpp
@@ -36,7 +36,10 @@
   std::string userKeyId;
 
   po::options_description description(
-    "Usage: ndnsec key-gen [-h] [-n] [-t TYPE] [-k KEYIDTYPE|--keyid KEYID] [-i] IDENTITY\n"
+    "Usage: ndnsec key-gen [-h] [-n] [-t TYPE] [-k KEYIDTYPE|--keyid KEYID]\n"
+    "                      [-i] IDENTITY\n"
+    "\n"
+    "Generate a public/private key pair for an identity.\n"
     "\n"
     "Options");
   description.add_options()