docs: also use term unsubstituted for migration guide

Co-authored-by: Milos Djermanovic <milos.djermanovic@gmail.com>

docs/src/use/migrate-to-9.0.0.md

@@ -552,6 +552,7 @@ In order to aid in the development of high-quality custom rules that are free fr
 1. **Suggestions must change the code.** Suggestions are expected to fix the reported problem by changing the code. `RuleTester` now throws an error if the suggestion test `output` is the same as the test `code`.
 1. **Suggestions must generate valid syntax.** In order for rule suggestions to be helpful, they need to be valid syntax. `RuleTester` now parses the output of suggestions using the same language options as the `code` value and throws an error if parsing fails.
 1. **Test cases must be unique.** Identical test cases can cause confusion and be hard to detect manually in a long test file. Duplicates are now automatically detected and can be safely removed.
+1. **Messages cannot have unsubstituted placeholders.** The rule tester now checks if there are `{{ placeholder }}` still in the message as they were not passed via `data` in the respective `context.report`.
 1. **`filename` and `only` must be of the expected type.** `RuleTester` now checks the type of `filename` and `only` properties of test objects. If specified, `filename` must be a string value. If specified, `only` must be a boolean value.
 
 **To address:** Run your rule tests using `RuleTester` and fix any errors that occur. The changes you'll need to make to satisfy `RuleTester` are compatible with ESLint v8.x.