docs(integ-tests): how to use `AwsApiCall.waiterProvider.addToRolePol…

…icy()` (aws#31167)

### Issue # (if applicable)

None

### Reason for this change

By using `AwsApiCall.provider.addToRolePolicy()`, we can add the necessary IAM policy to execute `AwsApiCall`. However, when using `waitForAssertions()`, the test will fail unless we use `AwsApiCall.waiterProvider.addToRolePolicy()`.
Since this information was not mentioned in the official documentation, I struggled to resolve the issue, so I submitted a pull request to add it to the documentation."

### Description of changes

Add description.

### Description of how you validated changes

None

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/integ-tests-alpha/README.md

@@ -345,6 +345,23 @@ apiCall.provider.addToRolePolicy({
 });
 ```
 
+When executing `waitForAssertion()`, it is necessary to add an IAM policy using `waiterProvider.addToRolePolicy()`.
+Because `IApiCall` does not have a `waiterProvider` property, you need to cast it to `AwsApiCall`.
+
+```ts
+declare const integ: IntegTest;
+
+const apiCall = integ.assertions.awsApiCall('S3', 'listObjectsV2', {
+  Bucket: 'mybucket',
+}).waitForAssertions() as AwsApiCall;
+
+apiCall.waiterProvider?.addToRolePolicy({
+  Effect: 'Allow',
+  Action: ['s3:GetObject', 's3:ListBucket'],
+  Resource: ['*'],
+});
+```
+
 Note that addToRolePolicy() uses direct IAM JSON policy blobs, not a iam.PolicyStatement
 object like you will see in the rest of the CDK.