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>