Document forbid-output for UI tests (#2171)

Author: clubby789

src/tests/directives.md

@@ -105,7 +105,7 @@ for more details.
 | `exec-env`                        | Env var to set when executing a test                                                                                     | `ui`, `crashes`                              | `<KEY>=<VALUE>`                                                                         |
 | `unset-exec-env`                  | Env var to unset when executing a test                                                                                   | `ui`, `crashes`                              | Any env var name                                                                        |
 | `stderr-per-bitwidth`             | Generate a stderr snapshot for each bitwidth                                                                             | `ui`                                         | N/A                                                                                     |
-| `forbid-output`                   | A pattern which must not appear in `cfail` output                                                                        | `incremental`                                | Regex pattern                                                                           |
+| `forbid-output`                   | A pattern which must not appear in stderr/`cfail` output                                                                 | `ui`, `incremental`                          | Regex pattern                                                                           |
 | `run-flags`                       | Flags passed to the test executable                                                                                      | `ui`                                         | Arbitrary flags                                                                         |
 | `known-bug`                       | No error annotation needed due to known bug                                                                              | `ui`, `crashes`, `incremental`               | Issue number `#123456`                                                                  |