Adding blurb about service endpoints

Adding blurb about service endpoints

Author: JackStromberg

articles/application-gateway/how-application-gateway-works.md

@@ -49,7 +49,7 @@ When an application gateway sends the original request to the backend server, it
 > If the backend pool:
 > - **Is a public endpoint**, the application gateway uses its frontend public IP to reach the server. If there isn't a frontend public IP address, one is assigned for the outbound external connectivity.
 > - **Contains an internally resolvable FQDN or a private IP address**, the application gateway routes the request to the backend server by using its instance private IP addresses.
-> - **Contains an external endpoint or an externally resolvable FQDN**, the application gateway routes the request to the backend server by using its frontend public IP address. The DNS resolution is based on a private DNS zone or custom DNS server, if configured, or it uses the default Azure-provided DNS. If there isn't a frontend public IP address, one is assigned for the outbound external connectivity.
+> - **Contains an external endpoint or an externally resolvable FQDN**, the application gateway routes the request to the backend server by using its frontend public IP address. If the subnet contains [service endpoints](../virtual-network/virtual-network-service-endpoints-overview.md), the application gateway will route the request to the service via its private IP address. DNS resolution is based on a private DNS zone or custom DNS server, if configured, or it uses the default Azure-provided DNS. If there isn't a frontend public IP address, one is assigned for the outbound external connectivity.
 
 ### Modifications to the request