Document dashboard variable functions like label_values, query_result, etc.

Today I figured out that you can use the function metrics and query_result as a dashboard variable. But I don’t see anything about this in the docs. I’m also missing docs about the other 2 possible functions: up and label_values. The way I found out: Go to example page: https://grafana.com/docs/grafana/latest/variables/variable-examples/ Click Prometheus templating - Uses chained query variables Inspect the vars on that demo dashbaord [Original issue](https://github.com/grafana/cloud-docs/issues/318) raised by @basvdl in the now archived grafana/cloud-docs repo