update mdx docs

Author: lawreka

api-playground/openapi/setup.mdx

@@ -96,9 +96,12 @@ only one OpenAPI file - it will automatically detect your OpenAPI file.
 <br />
 
 <Note>
-  The method and path must match the method and path specified in the OpenAPI
-  document exactly. If the endpoint doesn't exist in the OpenAPI file,
-  the page will be empty.
+  In most cases, the method and path must match the method and path specified
+  in the OpenAPI document exactly. If the endpoint doesn't exist in the OpenAPI
+  file, the page will be empty.
+
+  For webhooks, replace the method (i.e. "POST") with "webhook" (case insensitive)
+  and the correct method will be generated.
 </Note>
 
 ### Autogenerate files