security: Add doxygen comments

Change-Id: Ieafe4d13513e626d84ef9e621694f69844785018
diff --git a/src/security/certificate-subject-description.cpp b/src/security/certificate-subject-description.cpp
index 63e7b0b..ccf6778 100644
--- a/src/security/certificate-subject-description.cpp
+++ b/src/security/certificate-subject-description.cpp
@@ -63,4 +63,4 @@
   attributeTypeAndValue.MessageEnd();
 }
 
-}
+} // namespace ndn