Adjust some outdated docs for //@ forbid-output

Author: Zalathar

src/tests/compiletest.md

@@ -185,7 +185,7 @@ fn foo() {
 fn main() { foo(); }
 ```
 
-`cfail` tests support the `forbid-output` directive to specify that a certain
+Incremental tests support the `forbid-output` directive to specify that a certain
 substring must not appear anywhere in the compiler output.
 This can be useful to ensure certain errors do not appear, but this can be fragile as error messages
 change over time, and a test may no longer be checking the right thing but will still pass.

src/tests/directives.md

@@ -106,7 +106,7 @@ comparison](ui.md#output-comparison) and [Rustfix tests](ui.md#rustfix-tests) fo
 | `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 stderr/`cfail` output                                                                 | `ui`, `incremental`                          | Regex pattern                                                                           |
+| `forbid-output`                   | Check that compile/run output does not contain a specific string                                                         | `ui`, `incremental`                          | String                                                                                  |
 | `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`                                                                  |
 | `compare-output-by-lines`         | Compare the output by lines, rather than as a single string                                                              | All                                          | N/A                                                                                     |
@@ -315,8 +315,7 @@ See [Pretty-printer](compiletest.md#pretty-printer-tests).
 
 - `no-auto-check-cfg` — disable auto check-cfg (only for `--check-cfg` tests)
 - [`revisions`](compiletest.md#revisions) — compile multiple times
--[`forbid-output`](compiletest.md#incremental-tests) — incremental cfail rejects
-      output pattern
+- [`forbid-output`](compiletest.md#incremental-tests) — check that output does not contain a specified string
 - [`reference`] — an annotation linking to a rule in the reference
 - `disable-gdb-pretty-printers` — disable gdb pretty printers for debuginfo tests