proper comments in generated code

first of all, kudos to the great package/tool for code gen. it fits our stack of akka-http/circe perfectly.

small suggestion in readability of the generated code. some comments for the functions would be helpful, especially those the require mandatory implementation. For example, I have a use case of handling file uploads with `multipart/form-data`. I set `operationId: getUpload` and ended up with function I need to implement

```
getUpload(respond: UploadResource.getUploadResponse.type)(front: (File, Option[String], ContentType), back: (File, Option[String], ContentType)): scala.concurrent.Future[UploadResource.getUploadResponse]
```

```
getUploadMapFileField(fieldName: String, fileName: Option[String], contentType: ContentType): File
```

the first one is standard, but the second one took me a while to figure out. Some comments in the meaning of this function would be very helpful.