docs: explain that response parsing considers json_name

Author: diegomarquezp

sdk-platform-java/gax-java/gax-httpjson/src/main/java/com/google/api/gax/httpjson/ProtoRestSerializer.java

@@ -101,6 +101,9 @@ String toJson(Message message, boolean numericEnum) {
   @SuppressWarnings("unchecked")
   RequestT fromJson(Reader json, Message.Builder builder) {
     try {
+      // Supports mapping from both standard proto field names and explicit 'json_name' annotations.
+      // See:
+      // https://github.com/protocolbuffers/protobuf/blob/cecbbf41e43634c7c5b940dd336aa81b31fd4e5d/java/util/src/main/java/com/google/protobuf/util/JsonFormat.java#L1577-L1583
       JsonFormat.parser().usingTypeRegistry(registry).ignoringUnknownFields().merge(json, builder);
       return (RequestT) builder.build();
     } catch (IOException e) {