add server.listen docs for random port assignment

Documentation was missing the possibility of having server port randomly assigned.
"...empty value (...), or not specified, an ephemeral..." along with correctly specifying `port` as optional would cover what you are documenting as a different `.listner()` syntax.

I know there is a bit of a judgement call as to when a single method that does vastly different things (TCP versus local domain sockets, in this case) based on its argument types should be documented as a single method, or as multiple. In this case, my personal opinion is that having one listen() for TCP and one for Local domain makes more sense, then describing a missing  `port` for TCP as a different method.