docs(appsync): clarify the structure of apiKeys object (#33424)

### Issue # (if applicable)

n/a

### Reason for this change

I found it is not clear how api keys are stored in the `apiKey` object. e.g. how can we get an api key from the object?

### Description of changes

I added a description about the object structure from the below implementation.

https://github.com/aws/aws-cdk/blob/27e854f6c19048217b13c687ca7bfba2d69202a8/packages/aws-cdk-lib/aws-appsync/lib/eventapi.ts#L493

I confirmed this implementation also aligns with id naming rule. https://github.com/aws/aws-cdk/blob/9ae1d34b128a0a4332fb43ab2c5ccacbdb337a51/packages/aws-cdk-lib/aws-appsync/lib/auth-config.ts#L192



### Describe any new or updated permissions being added
none



### Description of how you validated changes
n/a


### 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: tmokmss

packages/aws-cdk-lib/aws-appsync/lib/eventapi.ts

@@ -414,7 +414,8 @@ export class EventApi extends EventApiBase {
   public readonly defaultSubscribeModeTypes: AppSyncAuthorizationType[];
 
   /**
-   * the configured API keys, if present
+   * The configured API keys, if present.
+   * The key of this object is an apiKey name (apiKeyConfig.name) if specified, `Default` otherwise.
    *
    * @default - no api key
    * @attribute ApiKeys