chore(ec2): improve doc for resolveSsmParameterAtLaunch() (#28110)

Improve the doc to clarify its limit.

Closes #28090

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packages/aws-cdk-lib/aws-ec2/README.md

@@ -1945,6 +1945,8 @@ const launchTemplate = new ec2.LaunchTemplate(this, 'LaunchTemplate', {
 });
 ```
 
+Please note this feature does not support Launch Configurations.
+
 ## Detailed Monitoring
 
 The following demonstrates how to enable [Detailed Monitoring](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-cloudwatch-new.html) for an EC2 instance. Keep in mind that Detailed Monitoring results in [additional charges](http://aws.amazon.com/cloudwatch/pricing/).