docs(readme): change undocumented params wording (#429)

README.md

@@ -472,12 +472,12 @@ response = client.post(
 print(response.headers.get("x-foo"))
 ```
 
-#### Undocumented params
+#### Undocumented request params
 
 If you want to explicitly send an extra param, you can do so with the `extra_query`, `extra_body`, and `extra_headers` request
 options.
 
-#### Undocumented properties
+#### Undocumented response properties
 
 To access undocumented response properties, you can access the extra fields like `response.unknown_prop`. You
 can also get all the extra fields on the Pydantic model as a dict with