docs(elasticloadbalancingv2): add comment to recommended policy. (#32379)

### Issue # (if applicable)

Closes #32307 .

### Reason for this change
Enum SslPolicy doesn't have a TLS13_12.
When I try to add, it was failed caused by the value conflicts with the
exiting value `RECOMMENDED_TLS`.
But User will be confused. So I add a comment.

#32377 (comment)

### Description of changes

add comment to doc. 

### Description of how you validated changes

<!--Have you added any unit tests and/or integration tests?-->

### 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)

----

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

---------

Co-authored-by: MasakiYamanaka <[email protected]>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: 3 people

packages/aws-cdk-lib/aws-elasticloadbalancingv2/lib/shared/enums.ts

@@ -105,6 +105,8 @@ export enum SslPolicy {
   /**
    * The recommended security policy for TLS listeners.
    * This is the default policy for listeners created using the AWS Management Console
+   *
+   * This policy includes TLS 1.3, and is backwards compatible with TLS 1.2
    */
   RECOMMENDED_TLS = 'ELBSecurityPolicy-TLS13-1-2-2021-06',