configuration-listeners.md: clarify exclusive port usage

clarify that the same port cannot be used for both public and private listeners at the same time.

also see the (quite concise)  FAQ 
https://learn.microsoft.com/en-us/azure/application-gateway/application-gateway-faq#can-i-use-the-same-port-for-both-public-facing-and-private-facing-listeners

Author: Josverl

articles/application-gateway/configuration-listeners.md

@@ -38,7 +38,7 @@ Choose the front-end IP address that you plan to associate with this listener. T
 
 ## Front-end port
 
-Choose the front-end port. Select an existing port or create a new one. Choose any value from the [allowed range of ports](./application-gateway-components.md#ports). You can use not only well-known ports, such as 80 and 443, but any allowed custom port that's suitable. A port can be used for public-facing listeners or private-facing listeners.
+Choose the front-end port. Select an existing port or create a new one. Choose any value from the [allowed range of ports](./application-gateway-components.md#ports). You can use not only well-known ports, such as 80 and 443, but any allowed custom port that's suitable. A port can be used for public-facing listeners or private-facing listeners, however the same port cannot be used for both at the same time.
 
 ## Protocol