Merge pull request #20840 from storybookjs/chore_docs_test_runner_add…

…_shard_option

Documentation: Add shard option to the test runner documentation

docs/writing-tests/test-runner.md

@@ -95,6 +95,7 @@ If you're already using any of those flags in your project, you should be able t
 | `-u`, `--updateSnapshot`        | Use this flag to re-record every snapshot that fails during this test run <br/>`test-storybook -u`                               |
 | `--eject`                       | Creates a local configuration file to override defaults of the test-runner <br/>`test-storybook --eject`                         |
 | `--coverage`                    | Runs [coverage tests](./test-coverage.md) on your stories and components <br/> `test-storybook --coverage`                       |
+| `--shard [index/count]`         | Requires CI. Splits the test suite execution into multiple machines <br/> `test-storybook --shard=1/8`                           |
 
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