[1704] Clarify why a null sync object is used in LoggerManager::readLocalMessageFile()

src/lib/log/logger_manager.cc

@@ -150,7 +150,10 @@ LoggerManager::readLocalMessageFile(const char* file) {
     MessageDictionary& dictionary = MessageDictionary::globalDictionary();
     MessageReader reader(&dictionary);
 
-    // Turn off use of any lock files
+    // Turn off use of any lock files. This is because this logger can
+    // be used by standalone programs which may not have write access to
+    // the local state directory (to create lock files). So we switch to
+    // using a null interprocess sync object here.
     logger.setInterprocessSync(new isc::util::InterprocessSyncNull("logger"));
 
     try {