text: Title: 404 Status Code not Docummented
  Body: ### Discussed in https://github.com/awtkns/fastapi-crudrouter/discussions/102
  
  <div type='discussions-op-text'>
  
  <sup>Originally posted by **sondrelg** September 16, 2021</sup>
  Hi!
  
  I was just testing out [schemathesis](https://github.com/schemathesis/schemathesis) on a new project implementing crudrouter, and found it complaining about the fact that one of my generated endpoints had undocumented response codes.
  
  Output:
  
  ```
  DELETE /api/v1/authors/{item_id} [P]
  1. Received a response with a status code, which is not defined in the schema: 404
  
  Declared status codes: 200, 422
  ```
  
  This error is _technically_ correct - there is no 404 documented, but I can definitely understand how crudrouter as a library might _not_ want to auto-document every possible status code, since it would make the schema way too detailed/verbose.
  
  I was just wondering, is it _possible_ to opt-into documenting extra status codes? Or is it possible to manually extend the status codes documented? 
  
  If this feature doesn't exist yet, do you think it would be something worth implementing? If nothing else it would be cool to be able to switch it on just for testing.</div>
  
  The aim of this feature is to document 404 status codes in the openapi spec. 