[DOCS] Add breaking change for unsupported `script` fields (#78217)

Adds an 8.0 breaking change for PR #59507.

docs/reference/migration/migrate_8_0/reindex.asciidoc

@@ -50,4 +50,17 @@ Similarly, the `size` parameter has been renamed to `max_docs` for
 Use the replacement parameters. Requests containing the `size` parameter will
 return an error.
 ====
+
+.The update by query API now rejects unsupported `script` fields.
+[%collapsible]
+====
+*Details* +
+An update by query API request that includes an unsupported field in the
+`script` object now returns an error. Previously, the API would silently ignore
+these unsupported fields.
+
+*Impact* +
+To avoid errors, remove unsupported fields from the `script` object.
+====
+
 //end::notable-breaking-changes[]