doc: clarify use of `0` port value

Clarify that using a port value of `0` will result in the operating
system identifying an available port for use.

PR-URL: https://github.com/nodejs/node/pull/7206
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: cjihrig - Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/http.md

@@ -532,8 +532,8 @@ a listener for the `'listening'` event.  See also [`net.Server.listen(path)`][].
 
 Begin accepting connections on the specified `port` and `hostname`. If the
 `hostname` is omitted, the server will accept connections on any IPv6 address
-(`::`) when IPv6 is available, or any IPv4 address (`0.0.0.0`) otherwise. A
-port value of zero will assign a random port.
+(`::`) when IPv6 is available, or any IPv4 address (`0.0.0.0`) otherwise. Use a
+port value of zero to have the operating system assign an available port.
 
 To listen to a unix socket, supply a filename instead of port and hostname.
 

doc/api/net.md

@@ -178,8 +178,8 @@ The parameter `backlog` behaves the same as in
 
 Begin accepting connections on the specified `port` and `hostname`. If the
 `hostname` is omitted, the server will accept connections on any IPv6 address
-(`::`) when IPv6 is available, or any IPv4 address (`0.0.0.0`) otherwise. A
-port value of zero will assign a random port.
+(`::`) when IPv6 is available, or any IPv4 address (`0.0.0.0`) otherwise. Use a
+port value of `0` to have the operating system assign an available port.
 
 Backlog is the maximum length of the queue of pending connections.
 The actual length will be determined by the OS through sysctl settings such as