prometheus: Improve IsAPIError's documentation (#29432)

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

pkg/tsdb/prometheus/prometheus.go

@@ -218,9 +218,9 @@ func parseResponse(value model.Value, query *PrometheusQuery) (*tsdb.QueryResult
 	return queryRes, nil
 }
 
+// IsAPIError returns whether err is or wraps a Prometheus error.
 func IsAPIError(err error) bool {
-	// Have to use errors.As to compare Prometheus errors, since errors.Is won't work due to Prometheus
-	// errors being pointers and errors.Is ends up comparing them by pointer address
+	// Check if the right error type is in err's chain.
 	var e *apiv1.Error
 	return errors.As(err, &e)
 }