doc: add docs for server.address() for pipe case

Add documentation for net.server.address() for the case it is listening
on a pipe or unix domain socket instead an IP socket.

PR-URL: https://github.com/nodejs/node/pull/12907
Fixes: https://github.com/nodejs/node/issues/12895
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

doc/api/net.md

@@ -58,11 +58,14 @@ added: v0.1.90
 -->
 
 Returns the bound address, the address family name, and port of the server
-as reported by the operating system.
+as reported by the operating system if listening on an IP socket.
 Useful to find which port was assigned when getting an OS-assigned address.
 Returns an object with `port`, `family`, and `address` properties:
 `{ port: 12346, family: 'IPv4', address: '127.0.0.1' }`
 
+For a server listening on a pipe or UNIX domain socket, the name is returned
+as a string.
+
 Example:
 
 ```js