add description about proxy protocol

[djshow832]

What is changed, added or deleted? (Required)

Refer to #13254. I was wrong about the behavior of proxy-protocol-networks. It actually accepts connections without Proxy Protocol for the addresses outside the list.

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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Oreoxmt
• ran-huang

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[Oreoxmt]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: d4eb30e

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #13419.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.6: #13420.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.0: #13421.