Improve documentation for subjectAltNames

[zioproto]

The user might be confused about the usage of subjectAltNames, trying to use it to add more hostnames to an endpoint.
Make clear that it requires TLS mode mutual, and the the alternative names are those presented by the client for authentication.

Originally proposed as a docs contribution:
istio/istio.io#15733
but the docs are autogenerated from this repo.

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[istio-testing]

Hi @zioproto. 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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[zioproto]

Cc: @nshankar13