docs: Improve docs for API Gateway int. response selection_pattern

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

@@ -61,5 +61,8 @@ The following arguments are supported:
 * `resource_id` - (Required) The API resource ID
 * `http_method` - (Required) The HTTP method (`GET`, `POST`, `PUT`, `DELETE`, `HEAD`, `OPTION`)
 * `status_code` - (Required) The HTTP status code
-* `selection_pattern` - (Optional) Specifies the regular expression pattern used to choose an integration response based on the response from the backend
+* `selection_pattern` - (Optional) Specifies the regular expression pattern used to choose
+  an integration response based on the response from the backend.
+  If the backend is an `AWS` Lambda function, the AWS Lambda function error header is matched.
+  For all other `HTTP` and `AWS` backends, the HTTP status code is matched.
 * `response_templates` - (Optional) A map specifying the templates used to transform the integration response body