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>