docs/configuring-playbook-prometheus-grafana.md: Add documentation on how to override the grafana fqn

docs/configuring-playbook-prometheus-grafana.md

@@ -56,6 +56,7 @@ Name | Description
 `matrix_nginx_proxy_proxy_synapse_metrics`|Set this to `true` to make matrix-nginx-proxy expose the Synapse metrics at `https://matrix.DOMAIN/_synapse/metrics`
 `matrix_nginx_proxy_proxy_synapse_metrics_basic_auth_enabled`|Set this to `true` to password-protect (using HTTP Basic Auth) `https://matrix.DOMAIN/_synapse/metrics` (the username is always `prometheus`, the password is defined in `matrix_nginx_proxy_proxy_synapse_metrics_basic_auth_key`)
 `matrix_nginx_proxy_proxy_synapse_metrics_basic_auth_key`|Set this to a password to use for HTTP Basic Auth for protecting `https://matrix.DOMAIN/_synapse/metrics` (the username is always `prometheus` - it's not configurable)
+`matrix_server_fqn_grafana`|Use this variable to override the Grafana web user-interface.
 
 
 ## More information