Update openapi.md (#1798)

* Update openapi.md

- Allow developers to avoid spending two hours of searching for the documentation which list the availables configurations behing openapi attribute

* fix(core/openapi.md): wording

Co-authored-by: Kévin Dunglas <[email protected]>

* fix(openapi.md): move explanation to appropriate section

---------

Co-authored-by: Kévin Dunglas <[email protected]>

Author: filozofer

core/openapi.md

@@ -108,6 +108,9 @@ The impact on the swagger-ui is the following:
 ## Using the OpenAPI and Swagger Contexts
 
 Sometimes you may want to change the information included in your OpenAPI documentation.
+
+For the full list of available configurations, please refer to the [OpenAPI Specifications](https://spec.openapis.org/oas/latest.html). The current doc page only gives some examples but focuses mostly on the OpenAPI integration inside API Platform without telling you all you can pass into the attributes.
+
 The following configuration will give you total control over your OpenAPI definitions:
 
 [codeSelector]