Prometheus: Add documentation for ad-hoc filters (grafana#31122)

* Prometheus: Add documentation for ad-hoc filters

* Update docs/sources/datasources/prometheus.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* Update docs/sources/datasources/prometheus.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* Add link to ad hoc filters doumentation

* Change direct url for relative

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

docs/sources/datasources/prometheus.md

@@ -141,6 +141,11 @@ There are two syntaxes:
 Why two ways? The first syntax is easier to read and write but does not allow you to use a variable in the middle of a word. When the _Multi-value_ or _Include all value_
 options are enabled, Grafana converts the labels from plain text to a regex compatible string. Which means you have to use `=~` instead of `=`.
 
+### Ad hoc filters variable
+
+Prometheus supports the special [ad hoc filters]({{< relref "../variables/variable-types/add-ad-hoc-filters.md" >}}) variable type. It allows you to specify any number of label/value filters on the fly. These filters are automatically
+applied to all your Prometheus queries.
+
 ## Annotations
 
 [Annotations]({{< relref "../dashboards/annotations.md" >}}) allow you to overlay rich event information on top of graphs. You add annotation