📝 Add OpenAPI warning to "Body - Fields" docs with extra schema exten…

…sions (#4846)

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>

docs/en/docs/tutorial/body-fields.md

@@ -57,6 +57,10 @@ You can declare extra information in `Field`, `Query`, `Body`, etc. And it will
 
 You will learn more about adding extra information later in the docs, when learning to declare examples.
 
+!!! warning
+    Extra keys passed to `Field` will also be present in the resulting OpenAPI schema for your application.
+    As these keys may not necessarily be part of the OpenAPI specification, some OpenAPI tools, for example [the OpenAPI validator](https://validator.swagger.io/), may not work with your generated schema.
+
 ## Recap
 
 You can use Pydantic's `Field` to declare extra validations and metadata for model attributes.