chore(lambda): improve the doc about the code signing in Lambda (#31500)

improve the doc about the code signing

### Issue # (if applicable)

Closes #29474

### Reason for this change

Singing is still required by user before the deployment. This PR clarifies this in the doc.

### Description of changes



### 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-lambda/README.md

@@ -1326,6 +1326,8 @@ When enabled, AWS Lambda checks every code deployment and verifies that the code
 For more information, see [Configuring code signing for AWS Lambda](https://docs.aws.amazon.com/lambda/latest/dg/configuration-codesigning.html).
 The following code configures a function with code signing.
 
+Please note the code will not be automatically signed before deployment. To ensure your code is properly signed, you'll need to conduct the code signing process either through the AWS CLI (Command Line Interface) [start-signing-job](https://awscli.amazonaws.com/v2/documentation/api/latest/reference/signer/start-signing-job.html) or by accessing the AWS Signer console.
+
 ```ts
 import * as signer from 'aws-cdk-lib/aws-signer';