(prometheus) add explanation about incompatibility of metric find query

docs/sources/datasources/prometheus.md

@@ -51,6 +51,8 @@ Name | Description
 
 For details of `metric names` & `label names`, and `label values`, please refer to the [Prometheus documentation](http://prometheus.io/docs/concepts/data_model/#metric-names-and-labels).
 
+> Note: The part of queries is incompatible with the version before 2.6, if you specify like `foo.*`, please change like `metrics(foo.*)`.
+
 You can create a template variable in Grafana and have that variable filled with values from any Prometheus metric exploration query.
 You can then use this variable in your Prometheus metric queries.