docs: openapi: reminders to the openapi spec

[tpoisseau]

• Explain how route.openapi() will integrate into the final schema
  • default values
  • what is auto-generated
  • how to describe the route
  • how to describe parameters (could be related to Need an efficient way to override/complete openapi auto-generated for params (qs / request body) #46)
  • remind parameters.$.in possible values (Webstorm in openapi.yaml suggest cookie, header, path, query)
  • how to describe return values (description of properties)
  • remind structure of response in schema
• Explain how tuyauConfig.openapi.documentation will integrate into the final schema
  • default values
  • what is auto-generated