drainer: add read-timeout configuration explanation

[lichunzhu]

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Add read-timeout configuration explanation

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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Oreoxmt
• shichun-0415

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[shichun-0415]

Rest LGTM

[Oreoxmt]

Rest LGTM

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #11683.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #11684.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #11685.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #11686.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #11687.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #11688.