StackSets "Specify regions" to list only enabled regions | Support for nested stacks using service-managed

### Name of the resource

AWS::CloudFormation::StackSet

### Resource name

_No response_

### Description

### StackSets
1. When selecting `Add all regions` under step 4 of deployment options `Specify regions`, it will be helpful to only add enabled regions and not all regions.
2. Currently, Nested Stack is not supported in SERVICE_MANAGED permission model, requesting support for Nested Stacks if using service-managed stackset.

### Other Details

_No response_

Start with the AWS::CloudFormation::StackSet description and the deployment-options step 4 reference in this issue. Clarify the expected behavior for filtering enabled regions and for nested stacks under the SERVICE_MANAGED permission model; done requires both requests to have defined, testable acceptance criteria.

Written by the indexing model from the issue text.