docs: add note from TrafficPolicy to PortTrafficPolicy as well

[yvespp]

This ensures that readers of the PortTrafficPolicy documentation understand that port-level settings do not inherit from destination-level settings.

Closes: istio/istio#58297

[istio-policy-bot]

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[istio-testing]

Hi @yvespp. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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