note on disabling public network access (hashicorp#27220)

website/docs/r/servicebus_namespace.html.markdown

@@ -99,6 +99,8 @@ A `network_rule_set` block supports the following:
 
 * `public_network_access_enabled` - (Optional) Whether to allow traffic over public network. Possible values are `true` and `false`. Defaults to `true`.
 
+-> **Note:** To disable public network access, you must also configure the property `public_network_access_enabled`.
+
 * `trusted_services_allowed` - (Optional) Are Azure Services that are known and trusted for this resource type are allowed to bypass firewall configuration? See [Trusted Microsoft Services](https://github.com/MicrosoftDocs/azure-docs/blob/master/articles/service-bus-messaging/includes/service-bus-trusted-services.md)
 
 * `ip_rules` - (Optional) One or more IP Addresses, or CIDR Blocks which should be able to access the ServiceBus Namespace.