Improve metrics docs page a bit

We mainly switch the anonymous metrics viewing variable
to false, along with other wording changes.

docs/configuring-playbook-prometheus-grafana.md

@@ -1,6 +1,6 @@
 # Enabling metrics and graphs for your Matrix server (optional)
 
-It can be useful to have some (visual) insight in the performance of your homeserver.
+It can be useful to have some (visual) insight into the performance of your homeserver.
 
 You can enable this with the following settings in your configuration file (`inventory/host_vars/matrix.<your-domain>/vars.yml`):
 
@@ -11,8 +11,13 @@ matrix_synapse_metrics_enabled: true
 matrix_prometheus_node_exporter_enabled: true
 
 matrix_grafana_enabled: true
-matrix_grafana_anonymous_access: true
+
+matrix_grafana_anonymous_access: false
+
 matrix_grafana_default_admin_user: yourname
+
+# Passwords containing special characters may be troublesome.
+# Changing the password subsequently won't work.
 matrix_grafana_default_admin_password: securelongpassword
 ```