Updated API GW integration response documentation

### Explanation for this change
Recently, I've been using Terraform to manage AWS API GWs with Lambda backends.
It appears that an explicit dependency is required. Not setting it would lead to this error:

```
[...] Error creating API Gateway Integration Response: NotFoundException: No integration defined for method
```

Thus, I found the thread below which exposes the problem too.

Relevant Terraform version: checked against 0.6.16
Thread issue: https://github.com/hashicorp/terraform/issues/6128

website/source/docs/providers/aws/r/api_gateway_integration_response.html.markdown

@@ -10,6 +10,9 @@ description: |-
 
 Provides an HTTP Method Integration Response for an API Gateway Resource.
 
+-> **Note:** Depends on having `aws_api_gateway_integration` inside your rest api. To ensure this
+you might need to add an explicit `depends_on` for clean runs.
+
 ## Example Usage
 
 ```