[aws-events] Clarify inputTransformer in docs

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-events-rule-target.html#cfn-events-rule-target-inputtransformer

I want to be able to use L2 Construct for InputTransformer

### Use Case

Closest I've found is this https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-events.CfnRule.InputTransformerProperty.html

Ideally I want to be able to use Rule https://docs.aws.amazon.com/cdk/api/latest/docs/aws-events-readme.html with InputTransformer instead of Input

### Proposed Solution

### Other

* [ ] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

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This is a :rocket: Feature Request

Start with the linked CloudFormation InputTransformer documentation and the AWS CDK Rule and CfnRule.InputTransformerProperty documentation. Compare the existing Rule Input API with the lower-level property, then determine the construct design and tests needed for Rule to support InputTransformer; done means the L2 Rule construct exposes and correctly synthesizes that configuration.

Written by the indexing model from the issue text.