Documented a breaking change regarding imports in @rails/ujs 7.1.0

Author: mguan2020

guides/source/upgrading_ruby_on_rails.md

@@ -257,6 +257,26 @@ See the [Testing Rails Applications](https://guides.rubyonrails.org/testing.html
 
 If you run a single file's tests (`bin/rails test test/models/user_test.rb`), `test:prepare` will not run before it.
 
+### Import syntax from `@rails/ujs` is modified
+
+Starting from Rails 7.1, the syntax for importing modules from `@rails/ujs` is modified. Rails no longer supports the
+direct import of a module from `@rails/ujs`. If users create and build a JavaScript file with the line `import
+{ fileInputSelector } from "@rails/ujs"`, they will encounter the error `export 'fileInputSelector'
+(imported as 'fileInputSelector') was not found in '@rails/ujs' (possible exports: default)`.
+
+In Rails 7.1, users should first import the Rails object directly from `@rails/ujs`.
+Users can then import specific modules from the Rails object.
+
+An example of imports in Rails 7.1 is shown below:
+
+```javascript
+import Rails from "@rails/ujs"
+// Alias the method
+const fileInputSelector = Rails.fileInputSelector
+// Alternatively, reference it from the Rails object where it is used
+Rails.fileInputSelector(...)
+```
+
 ### `ActionView::TestCase#rendered` no longer returns a `String`
 
 Starting from Rails 7.1, `ActionView::TestCase#rendered` returns an object that