common.hpp: mark as implementation detail

refs #2098

Change-Id: Ibe8690eab5663be961b503d57932d5a59a57b54c
diff --git a/src/common.hpp b/src/common.hpp
index 551e8eb..4caf11b 100644
--- a/src/common.hpp
+++ b/src/common.hpp
@@ -19,6 +19,12 @@
  * See AUTHORS.md for complete list of ndn-cxx authors and contributors.
  */
 
+/** \file
+ *  \brief import common constructs for ndn-cxx library internal use
+ *  \warning This file is implementation detail of ndn-cxx library.
+ *           Aliases imported in this file SHOULD NOT be used outside of ndn-cxx.
+ */
+
 #ifndef NDN_COMMON_HPP
 #define NDN_COMMON_HPP