Add properties file to a comment listing all the source of configuration

Author: labbati

dd-trace-api/src/main/java/datadog/trace/api/Config.java

@@ -195,7 +195,7 @@ public enum PropagationStyle {
   // Values from an optionally provided properties file
   private static Properties propertiesFromConfigFile;
 
-  // Read order: System Properties -> Env Variables, [-> default value]
+  // Read order: System Properties -> Env Variables, [-> properties file], [-> default value]
   // Visible for testing
   Config() {
     propertiesFromConfigFile = loadConfigurationFile();