Add more elaborate docs regarding label shorthand

doc/user/project/integrations/prometheus.md

@@ -313,7 +313,7 @@ metrics:
     unit: "count"
 ```
 
-This will render into:
+This works by lowercasing the value of `label` and, if there are more words separated by spaces, replacing those spaces with an underscore (`_`). The transformed value is then checked against the labels of the time series returned by the Prometheus query. If a time series label is found that is equal to the transformed value, then the label value will be used and rendered in the legend like this:
 
 ![legend with label shorthand variable](img/prometheus_dashboard_label_variable_shorthand.png)