doc: add defaults option example to cluster documentation (#2377)

* doc: add defaults option example to cluster documentation

* Formatting update.

* Formatting.

Co-authored-by: Simon Prickett <simon@redis.com>

Author: yangbodong22011

docs/clustering.md

@@ -40,6 +40,22 @@ const value = await cluster.get('key');
 | modules                |         | Included [Redis Modules](../README.md#packages)                                                                                                                                                                                                                                                                     |
 | scripts                |         | Script definitions (see [Lua Scripts](../README.md#lua-scripts))                                                                                                                                                                                                                                                    |
 | functions              |         | Function definitions (see [Functions](../README.md#functions))                                                                                                                                                                                                                                                      |
+## Auth with password and username
+
+Specifying the password in the URL or a root node will only affect the connection to that specific node. In case you want to set the password for all the connections being created from a cluster instance, use the `defaults` option.
+```javascript
+createCluster({
+  rootNodes: [{
+    url: 'redis://10.0.0.1:30001'
+  }, {
+    url: 'redis://10.0.0.2:30002'
+  }],
+  defaults: {
+    username: 'username',
+    password: 'password'
+  }
+});
+```
 
 ## Node Address Map