chunks: improve help text of ndnputchunks

refs #3772

Change-Id: I17c3be0cca53904f160c61893abac28edfc73134
diff --git a/tools/chunks/putchunks/ndnputchunks.cpp b/tools/chunks/putchunks/ndnputchunks.cpp
index 202cd83..195d2ca 100644
--- a/tools/chunks/putchunks/ndnputchunks.cpp
+++ b/tools/chunks/putchunks/ndnputchunks.cpp
@@ -28,9 +28,19 @@
 #include "core/version.hpp"
 #include "producer.hpp"
 
+namespace po = boost::program_options;
+
 namespace ndn {
 namespace chunks {
 
+static void
+usage(std::ostream& os, const std::string& programName, const po::options_description& visibleDesc) {
+  os << "Usage: " << programName << " [options] ndn:/name" << std::endl;
+  os << "\nPublish data under specified prefix. "
+     << "Note: this tool expects data from the standard input.\n" << std::endl;
+  os << visibleDesc;
+}
+
 static int
 main(int argc, char** argv)
 {
@@ -42,7 +52,6 @@
   bool isVerbose = false;
   std::string prefix;
 
-  namespace po = boost::program_options;
   po::options_description visibleDesc("Options");
   visibleDesc.add_options()
     ("help,h",          "print this help message and exit")
@@ -82,8 +91,7 @@
   }
 
   if (vm.count("help") > 0) {
-    std::cout << "Usage: " << programName << " [options] ndn:/name" << std::endl;
-    std::cout << visibleDesc;
+    usage(std::cout, programName, visibleDesc);
     return 0;
   }
 
@@ -93,8 +101,7 @@
   }
 
   if (prefix.empty()) {
-    std::cerr << "Usage: " << programName << " [options] ndn:/name" << std::endl;
-    std::cerr << visibleDesc;
+    usage(std::cerr, programName, visibleDesc);
     return 2;
   }