Improve guide on listening on privileged ports

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guide/listeners.md

@@ -99,12 +99,19 @@ argument is the name of the listener you gave in `ranch:start_listener/6`.
 Port = ranch:get_port(tcp_echo).
 ```
 
-Listening on a port =< 1024
----------------------------
+Listening on privileged ports
+-----------------------------
 
-This is currently not possible. We recommend the use of load balancing
-or NAT firewall rules if the need arise. Proxies can sometimes also be
-used although that's a less efficient solution.
+Some systems limit access to ports below 1024 for security reasons.
+This can easily be identified by an `{error, eacces}` error when trying
+to open a listening socket on such a port.
+
+The methods for listening on privileged ports vary between systems,
+please refer to your system's documentation for more information.
+
+We recommend the use of port rewriting for systems with a single server,
+and load balancing for systems with multiple servers. Documenting these
+solutions is however out of the scope of this guide.
 
 Limiting the number of concurrent connections
 ---------------------------------------------

guide/toc.md

@@ -8,7 +8,7 @@ Ranch User Guide
    *  Purpose
    *  Starting and stopping
    *  Listening on a random port
-   *  Listening on a port =< 1024
+   *  Listening on privileged ports
    *  Limiting the number of concurrent connections
    *  Upgrading
  *  [Transports](transports.md)