docs(rustdoc): document that JS/TS checks run under tidy (eslint, es-check, tsc)

Author: ali90h

src/rustdoc.md

@@ -60,6 +60,18 @@ does is call the `main()` that's in this crate's `lib.rs`, though.)
 * Use `./x test tests/rustdoc*` to run the tests using a stage1
   rustdoc.
   * See [Rustdoc internals] for more information about tests.
+* Use `./x.py test tidy --extra-checks=js` to run rustdoc’s JavaScript checks (`eslint`, `es-check`, and `tsc`).
+> **Note:** `./x.py test tidy` already runs these checks automatically when JS/TS sources changed; `--extra-checks=js` forces them explicitly.
+
+### JavaScript CI checks
+
+Rustdoc’s JavaScript and TypeScript are checked during CI by `eslint`, `es-check`, and `tsc` (not by compiletest). These run as part of the `tidy` job.
+
+```bash
+./x.py test tidy --extra-checks=js
+```
+
+The `--extra-checks=js` flag enables the frontend linting that runs in CI.
 
 [`bootstrap.toml`]: ./building/how-to-build-and-run.md