Update 3.x upgrade guide.md (#2445)

Clarify based on #2436 (comment)

Author: epicfaace

docs/3.x upgrade guide.md

@@ -12,7 +12,7 @@ Dropped support for Node 8, 9, 10. Minimum supported version of Node.js is now 1
 
 ### Help field IDs
 
-IDs for [Help fields](https://react-jsonschema-form.readthedocs.io/en/latest/api-reference/uiSchema/#help) are now suffixed by `__help` so that the IDs are unique. Previously, their IDs would be the same as the fields that they were describing.
+IDs for [Help fields](https://react-jsonschema-form.readthedocs.io/en/latest/api-reference/uiSchema/#help) are now suffixed by `__help` so that the IDs are unique. Previously, their IDs would be nonexistent or the same as the fields that they were describing.
 
 ### Bring your own polyfills