Updated API GW integration response documentation
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
terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,ashinohara/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,terraform-providers/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ClementCunin/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,ashinohara/terraform-provider-aws,ClementCunin/terraform-provider-aws,ashinohara/terraform-provider-aws