docs: Updates dgram doc with optional port for socket.bind()

[dcousens]

As explained several times throughout this document already:

socket.bind() will bind to the "all interfaces" address on a random port (it does the right thing for both udp4 and udp6 sockets).

and

If the socket has not been previously bound with a call to bind, it's assigned a random port number and bound to the "all interfaces" address (0.0.0.0 for udp4 sockets, ::0 for udp6 sockets).

I felt for an API lookup, it is a good idea to repeat this in socket.bind itself.

[Nodejs-Jenkins]

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• Daniel Cousens

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[bnoordhuis]

Long line, please wrap at 80 columns.

[bnoordhuis]

Also, the "does the right thing" comment elsewhere applies to the address, not the port.

[dcousens]

Is what exists now suitable? (See latest commit)