chore(core): Explain usage of '--fail' in 'cdk diff' (#25638)

Explain usage of '--fail' in 'cdk diff'. See https://stackoverflow.com/questions/69574034/what-are-the-returned-status-code-of-cdk-diff

----

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

Author: markusl

packages/aws-cdk/README.md

@@ -146,8 +146,8 @@ See the [CDK reference documentation](https://docs.aws.amazon.com/cdk/api/latest
 ### `cdk diff`
 
 Computes differences between the infrastructure specified in the current state of the CDK app and the currently
-deployed application (or a user-specified CloudFormation template). This command returns non-zero if any differences are
-found.
+deployed application (or a user-specified CloudFormation template). If you need the command to return a non-zero if any differences are
+found you need to use the `--fail` command line option.
 
 ```console
 $ # Diff against the currently deployed stack