doc: correct and complete dgram's Socket.bind docs

`port` was listed as required, but as described in the following
paragraphs, it's actually not.

Also, note that setting `port` to `0` will also cause the OS to assign a
a random port and sync up the docs of both forms.

PR-URL: https://github.com/nodejs/node/pull/11025


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>