docs(custom-resource): add example for AwsApiCall (#26621)

There weren't any examples for `AwsApiCall` in the document other than the Overview.
This commit adds examples to the document of  `AwsApiCall` and it helps users understand the usage.

----

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packages/aws-cdk-lib/custom-resources/lib/aws-custom-resource/aws-custom-resource.ts

@@ -74,6 +74,24 @@ export class PhysicalResourceId {
 
 /**
  * An AWS SDK call.
+ *
+ * @example
+ *
+ *    new cr.AwsCustomResource(this, 'GetParameterCustomResource', {
+ *      onUpdate: { // will also be called for a CREATE event
+ *        service: 'SSM',
+ *        action: 'getParameter',
+ *        parameters: {
+ *          Name: 'my-parameter',
+ *          WithDecryption: true,
+ *        },
+ *        physicalResourceId: cr.PhysicalResourceId.fromResponse('Parameter.ARN'),
+ *      },
+ *      policy: cr.AwsCustomResourcePolicy.fromSdkCalls({
+ *        resources: cr.AwsCustomResourcePolicy.ANY_RESOURCE,
+ *      }),
+ *    });
+ *
  */
 export interface AwsSdkCall {
   /**