service/prometheus.md

@@ -662,6 +662,19 @@ depending on the Alertmanager routing. The `summary` field ends up in
 the `Subject` field of outgoing email, and the `description` is the
 email body, for example.
 
+Those fields are [Golang templates][] with variables accessible with
+curly braces. For example, `{{ $value }}` is the actual value of the
+metric in the `expr` query. The list of available variables is
+somewhat obscure, but some of it is visible in the [Prometheus
+template reference][] and the [Alertmanager template reference][]. The
+Golang templating system also comes with its own [limited set of
+built-in functions][].
+
+ [Prometheus template reference]: https://prometheus.io/docs/prometheus/latest/configuration/template_reference/
+ [Alertmanager template reference]: https://prometheus.io/docs/alerting/latest/notifications/
+ [limited set of built-in functions]: https://pkg.go.dev/text/template#hdr-Functions
+ [Golang templates]: https://pkg.go.dev/text/template
+
 ### Writing a playbook
 
 Every alert in Prometheus *must* have a playbook annotation. This is