chore(kinesisfirehose): update README to explicitly clarify which sources/destinations are supported in the CDK (#31868)

### Description of changes
update README to explicitly clarify which sources/destinations are supported in the CDK

### 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*

Author: paulhcsun

packages/@aws-cdk/aws-kinesisfirehose-alpha/README.md

@@ -54,7 +54,7 @@ The above example defines the following resources:
 
 ## Sources
 
-An Amazon Data Firehose delivery stream can accept data from three main sources: Kinesis Data Streams, Managed Streaming for Apache Kafka (MSK), or via a "direct put" (API calls).
+An Amazon Data Firehose delivery stream can accept data from three main sources: Kinesis Data Streams, Managed Streaming for Apache Kafka (MSK), or via a "direct put" (API calls). Currently only Kinesis Data Streams and direct put are supported in the CDK. 
 
 See: [Sending Data to a Delivery Stream](https://docs.aws.amazon.com/firehose/latest/dev/basic-write.html)
 in the *Amazon Data Firehose Developer Guide*.
@@ -96,7 +96,9 @@ Data must be provided via "direct put", ie., by using a `PutRecord` or
 
 ## Destinations
 
-The following destinations are supported. See [kinesisfirehose-destinations](https://docs.aws.amazon.com/cdk/api/latest/docs/aws-kinesisfirehose-destinations-readme.html)
+Amazon Data Firehose supports multiple AWS and third-party services as destinations, including Amazon S3, Amazon Redshift, and more. You can find the full list of supported destination [here](https://docs.aws.amazon.com/firehose/latest/dev/create-destination.html). 
+
+Currently in the AWS CDK, only S3 is implemented as an L2 construct destination. Other destinations can still be configured using L1 constructs. See [kinesisfirehose-destinations](https://docs.aws.amazon.com/cdk/api/latest/docs/aws-kinesisfirehose-destinations-readme.html)
 for the implementations of these destinations.
 
 ### S3