Added docstring to ns_bind() and ns_bind_opt()

What about `[PROTO://][IP_ADDRESS][:PORT]` (and disallowing just a port number) ?

Rationale: try to open http://918700538 in your browser or with curl. Same for ping, telnet and other utils.

We currently do parse ":8080" and "udp://:53" (in addition to plain numeric port numbers, see https://github.com/cesanta/fossa/blob/master/test/unit_test.c#L1781)

Also golang uses this convention to bind to any address (see http://golang.org/pkg/net/)

It might be useful to provide a variant of the `ns_bind` call (perhaps via the `opts` that receives the port as an `int`, to help people open many consecutive ports without having to stringify them first).