…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)

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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*