chore: adding some explanations about setting prometheus password

docs/configuring-playbook-prometheus-grafana.md

@@ -55,7 +55,7 @@ Name | Description
 `matrix_synapse_metrics_enabled`|Set this to `true` to make Synapse expose metrics (locally, on the container network)
 `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_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).Do not write the password in plain text. See `man 1 htpasswd` or use `htpasswd -c mypass.htpasswd prometheus` to generate the expected hash for nginx. 
 `matrix_server_fqn_grafana`|Use this variable to override the domain at which the Grafana web user-interface is at (defaults to `stats.DOMAIN`)
 
 ### Collecting worker metrics to an external Prometheus server