docs(opensearch): add example for token based import (#20915)

Documentation for changes made in PR #15219 - issue #15188

Normally one would not put the domain endpoint as a static value in a CF stack.
A more realistic use case is to manage an opensearch domain from CF stack A and use it via CF stack B, C, D ...
Stack A would export the domainEndpoint and also domainArn - the other stacks can then import those values and import the domain.

This PR will add an example for the realistic scenario so people do not end up with errors like "Invalid URL" when a token endpoint is provided.



----

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### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: akleiber

packages/@aws-cdk/aws-opensearchservice/README.md

@@ -97,6 +97,8 @@ const slr = new iam.CfnServiceLinkedRole(this, 'Service Linked Role', {
 
 ## Importing existing domains
 
+### Using a known domain endpoint
+
 To import an existing domain into your CDK application, use the `Domain.fromDomainEndpoint` factory method.
 This method accepts a domain endpoint of an already existing domain:
 
@@ -105,6 +107,20 @@ const domainEndpoint = 'https://my-domain-jcjotrt6f7otem4sqcwbch3c4u.us-east-1.e
 const domain = opensearch.Domain.fromDomainEndpoint(this, 'ImportedDomain', domainEndpoint);
 ```
 
+### Using the output of another CloudFormation stack
+
+To import an existing domain with the help of an exported value from another CloudFormation stack,
+use the `Domain.fromDomainAttributes` factory method. This will accept tokens.
+
+```ts
+const domainArn = Fn.importValue(`another-cf-stack-export-domain-arn`);
+const domainEndpoint = Fn.importValue(`another-cf-stack-export-domain-endpoint`);
+const domain = Domain.fromDomainAttributes(this, 'ImportedDomain', {
+  domainArn,
+  domainEndpoint,
+});
+```
+
 ## Permissions
 
 ### IAM