add openapi diff note

Author: isabellaenriquez

develop-docs/backend/api/public.mdx

@@ -412,6 +412,8 @@ will be triggered to make your changes automatically go live:
 
 ## Troubleshooting API Docs Tests
 
+NOTE: The `openapi-diff` test is supposed to fail when CI runs on your pull request as it is meant to highlight the schema changes. It is not required to merge.
+
 **Problem**: `TypeError: Cannot read properties of null (reading 'type')`
 
 **Solution**: If you have a `ChoiceField` that can be null, make sure you're using the right option. `allow_blank` should be used for textual choices, while `allow_null` should be used for numeric and other non-textual choices.