add server.listen docs for random port assignment

Documentation was missing the possibility of having server port randomly assigned.
Do not use random to describe this port, the correct term is "ephemeral".

The docs are incorrect: they omit `hostname`, `backlog`, and `callback` arguments.

What you are observing is that in `server.listen(port, [hostname], [backlog], [callback])`, if the port is 0 (or anything that converts to `0`, such as `"0"`, `""`, `null`, or `undefined`), an ephemeral port is allocated.

You should modify the `server.listen(port)` docs to state that an ephemeral port is allocated if the `port` argument converts to `0`.