docs(event-targets): clarify that adding an imported SNS Topic as an …

…event target does not set required permissions (aws#30279)

### Issue aws#25583 

Closes aws#25583

### Reason for this change

Add documentation to clarify that imported topics have to have the required permissions set manually.

### Description of changes

Added docstring.

### Description of how you validated changes



### 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)

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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*

packages/aws-cdk-lib/aws-events-targets/lib/sns.ts

@@ -17,6 +17,7 @@ export interface SnsTopicProps extends TargetBaseProps {
 
 /**
  * Use an SNS topic as a target for Amazon EventBridge rules.
+ * If the topic is imported the required permissions to publish to that topic need to be set manually.
  *
  * @example
  *   /// fixture=withRepoAndTopic