Add additional documentation about Prometheus + Templated queries

docs/sources/datasources/prometheus.md

@@ -72,4 +72,8 @@ label_values(hostname)
 
 You can also use raw queries & regular expressions to extract anything you might need.
 
+### Using templated variables in queries
+
+When the `Include All` option or `Multi-Value` option is enabled, Grafana converts the labels from plain text to a regex compatible string. If your labels include special values such as `.` or `/`, you will need to use the `=~` or regex match in your Prometheus query. For example, a query for ALERTS, a template query of `label(ALERTS,instance)` and an instance value of `http://example.com` would become `ALERTS{instance=~$instance}` instead of `ALERTS{instance=$instance}`.
+
 ![](/img/v2/prometheus_templating.png)