update alertmanager data source docs (#41040)

docs/sources/datasources/alertmanager.md

@@ -8,9 +8,11 @@ weight = 150
 
 # Alertmanager data source
 
-Grafana includes built-in support for Prometheus Alertmanager. It is presently in alpha and not accessible unless [alpha plugins are enabled in Grafana settings](https://grafana.com/docs/grafana/latest/administration/configuration/#enable_alpha). Once you add it as a data source, you can use the [Grafana alerting UI](https://grafana.com/docs/grafana/latest/alerting/) to manage silences, contact points as well as notification policies. A drop-down option in these pages allows you to switch between Grafana and any configured Alertmanager data sources .
+Grafana includes built-in support for Prometheus Alertmanager. It is presently in alpha and not accessible unless [alpha plugins are enabled in Grafana settings](https://grafana.com/docs/grafana/latest/administration/configuration/#enable_alpha). Once you add it as a data source, you can use the [Grafana alerting UI](https://grafana.com/docs/grafana/latest/alerting/) to manage silences, contact points as well as notification policies. A drop-down option in these pages allows you to switch between Grafana and any configured Alertmanager data sources.
 
-> **Note:** Currently, the [Cortex implementation of Prometheus Alertmanager](https://cortexmetrics.io/docs/proposals/scalable-alertmanager/) is required to edit rules.
+## Alertmanager implementations
+
+[Prometheus](https://prometheus.io/) and [Cortex](https://cortexmetrics.io/) (default) implementations of Alertmanager are supported. You can specify implementation in the data source settings page. In case of Prometheus contact points and notification policies are read-only in the Grafana alerting UI, as it does not support updating configuration via HTTP API.
 
 ## Provision the Alertmanager data source
 
@@ -27,7 +29,7 @@ datasources:
     url: http://localhost:9090
     access: proxy
     jsonData:
-      implementation: 'prometheus'
+      implementation: 'prometheus' # alternatively 'cortex'
     # optionally
     basicAuth: true
     basicAuthUser: my_user