mention --edition restrictions in rustc-dev-guide

Author: pietroalbini

Diff for: src/doc/rustc-dev-guide/src/tests/directives.md

@@ -229,14 +229,14 @@ ignoring debuggers.
 
 ### Affecting how tests are built
 
-| Directive           | Explanation                                                                                  | Supported test suites     | Possible values                                                              |
-|---------------------|----------------------------------------------------------------------------------------------|---------------------------|------------------------------------------------------------------------------|
-| `compile-flags`     | Flags passed to `rustc` when building the test or aux file                                   | All except for `run-make` | Any valid `rustc` flags, e.g. `-Awarnings -Dfoo`. Cannot be `-Cincremental`. |
-| `edition`           | Alias for `compile-flags: --edition=xxx`                                                     | All except for `run-make` | Any valid `--edition` value                                                  |
-| `rustc-env`         | Env var to set when running `rustc`                                                          | All except for `run-make` | `<KEY>=<VALUE>`                                                              |
-| `unset-rustc-env`   | Env var to unset when running `rustc`                                                        | All except for `run-make` | Any env var name                                                             |
-| `incremental`       | Proper incremental support for tests outside of incremental test suite                       | `ui`, `crashes`           | N/A                                                                          |
-| `no-prefer-dynamic` | Don't use `-C prefer-dynamic`, don't build as a dylib via a `--crate-type=dylib` preset flag | `ui`, `crashes`           | N/A                                                                          |
+| Directive           | Explanation                                                                                  | Supported test suites     | Possible values                                                                            |
+|---------------------|----------------------------------------------------------------------------------------------|---------------------------|--------------------------------------------------------------------------------------------|
+| `compile-flags`     | Flags passed to `rustc` when building the test or aux file                                   | All except for `run-make` | Any valid `rustc` flags, e.g. `-Awarnings -Dfoo`. Cannot be `-Cincremental` or `--edition` |
+| `edition`           | The edition used to build the test                                                           | All except for `run-make` | Any valid `--edition` value                                                                |
+| `rustc-env`         | Env var to set when running `rustc`                                                          | All except for `run-make` | `<KEY>=<VALUE>`                                                                            |
+| `unset-rustc-env`   | Env var to unset when running `rustc`                                                        | All except for `run-make` | Any env var name                                                                           |
+| `incremental`       | Proper incremental support for tests outside of incremental test suite                       | `ui`, `crashes`           | N/A                                                                                        |
+| `no-prefer-dynamic` | Don't use `-C prefer-dynamic`, don't build as a dylib via a `--crate-type=dylib` preset flag | `ui`, `crashes`           | N/A                                                                                        |
 
 <div class="warning">
 Tests (outside of `run-make`) that want to use incremental tests not in the