Stack instances Status "pending" is misleading

### Name of the resource

AWS::CloudFormation::StackSet

### Resource name

StackSet instance status

### Description

When a stackset is updated where in only a selective stack instances are updated, the stack instances that are not part of the deployment will go into Status: OUTDATED, DetailedStatus: PENDING .

These statuses are misleading because as per the CloudFormation [Stack instance status codes](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/stacksets-concepts.html#stack-instance-status-codes) ,

> PENDING The operation in the specified account and Region has yet to start.

However, there is nothing pending. This causes confusion and limits us from being able to verify the actual status from the StackSet's Stack instances without having to go to the actual stack.

Redeploying the stack instances would update the status to SUCCEEDED, however, this is not feasible when you have close to 10,000 stacks.

Please see if you can improve the status to provide something more meaningful.

### Other Details

_No response_

Start by reviewing the AWS CloudFormation StackSet instance status codes linked in the issue, focusing on OUTDATED and PENDING during selective deployments. Confirm the behavior for instances excluded from an update and determine what status would accurately represent that state; done means the reported status is no longer misleading without requiring redeployment of all instances.

Written by the indexing model from the issue text.