docs(rds): clarify how to enable S3Import/S3Export in Readme.md (#30459)

### Reason for this change

Documentation does not state clearly that feature needs to be manually enabled when using Postgres

Example : 
The following code will fail with the error message 
```ts
const cluster = new rds.DatabaseCluster(this, 'Database', {
      engine: rds.DatabaseClusterEngine.auroraPostgres({version: AuroraPostgresEngineVersion.of('12.7','12')}),
      writer: rds.ClusterInstance.provisioned('writer', {
        instanceType: ec2.InstanceType.of(ec2.InstanceClass.R6G, ec2.InstanceSize.XLARGE4),
      }),
      serverlessV2MinCapacity: 6.5,
      serverlessV2MaxCapacity: 64,
      s3ImportRole: sampleRole
});
```
Will fail with this error message :`Error: s3Import is not supported for Postgres version: 12.7. Use a version that supports the s3Import feature`

But the thing is that 12.7 does support it ! The real problem here is that the feature needs to be enabled as such

```ts
const cluster = new rds.DatabaseCluster(this, 'Database', {
      engine: rds.DatabaseClusterEngine.auroraPostgres({version: AuroraPostgresEngineVersion.of('12.7','12', {s3Export: true, s3Import:true})}),
      writer: rds.ClusterInstance.provisioned('writer', {
        instanceType: ec2.InstanceType.of(ec2.InstanceClass.R6G, ec2.InstanceSize.XLARGE4),
      }),
      serverlessV2MinCapacity: 6.5,
      serverlessV2MaxCapacity: 64,
      s3ImportRole: sampleRole
});
```
### Description of changes

Small update to Readme.md and inline TSdoc

### Description of how you validated changes
None

### 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*

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

@@ -938,6 +938,7 @@ Data in S3 buckets can be imported to and exported from certain database engines
 functionality, set the `s3ImportBuckets` and `s3ExportBuckets` properties for import and export respectively. When
 configured, the CDK automatically creates and configures IAM roles as required.
 Additionally, the `s3ImportRole` and `s3ExportRole` properties can be used to set this role directly.
+Note: To use `s3ImportRole` and `s3ExportRole` with Aurora PostgreSQL, you must also enable the S3 import and export features when you create the DatabaseClusterEngine.
 
 You can read more about loading data to (or from) S3 here:
 

packages/aws-cdk-lib/aws-rds/lib/cluster.ts

@@ -253,7 +253,7 @@ interface DatabaseClusterBaseProps {
    * This feature is only supported by the Aurora database engine.
    *
    * This property must not be used if `s3ImportBuckets` is used.
-   *
+   * To use this property with Aurora PostgreSQL, it must be configured with the S3 import feature enabled when creating the DatabaseClusterEngine
    * For MySQL:
    * @see https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/AuroraMySQL.Integrating.LoadFromS3.html
    *
@@ -284,7 +284,7 @@ interface DatabaseClusterBaseProps {
    * This feature is only supported by the Aurora database engine.
    *
    * This property must not be used if `s3ExportBuckets` is used.
-   *
+   * To use this property with Aurora PostgreSQL, it must be configured with the S3 export feature enabled when creating the DatabaseClusterEngine
    * For MySQL:
    * @see https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/AuroraMySQL.Integrating.SaveIntoS3.html
    *