Add doc about the multiple named ports

[jayunit100]

We should updates
https://github.com/kubernetes-sigs/kpng/blob/master/doc/service-proxy.md

With the learnings from #532

/good-first-issue

[k8s-ci-robot]

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In response to this:

We should updates
https://github.com/kubernetes-sigs/kpng/blob/master/doc/service-proxy.md

With the learnings from #532

/good-first-issue

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