test_runner: added docs

doc/api/cli.md

@@ -1500,6 +1500,26 @@ changes:
 Configures the test runner to only execute top level tests that have the `only`
 option set.
 
+### `--shards`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+Test suite shard to execute in a format of `<index>/<total>`, where
+
+`index` is a positive integer, index of divided parts
+`total` is a positive integer, total of divided part
+This command will divide all tests files into `total` equal parts, and will run only those that happen to be in an `index` part.
+
+For example, to split your tests suite into three parts, use this:
+
+```sh
+node --test --shard=1/3
+node --test --shard=2/3
+node --test --shard=3/3
+```
+
 ### `--throw-deprecation`
 
 <!-- YAML

doc/api/test.md

@@ -851,6 +851,11 @@ changes:
     If unspecified, subtests inherit this value from their parent.
     **Default:** `Infinity`.
   * `watch` {boolean} Whether to run in watch mode or not. **Default:** `false`.
+  * `shards` {Object} Running tests in a specific shard. **Default:** `undefined`.
+    * `index` {number} is a positive integer between 1 and `<total>` that specifies the index of the shard to run.
+      This option is _required_.
+    * `total` {number} is a positive that specifies the total number of shards to split the test files to.
+      This option is _required_.
 * Returns: {TestsStream}
 
 ```mjs