txn: update the document about read-consistency read check ts variable

[TomShawn]

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• This PR is translated from: txn: update the document about read-consistency read check ts variable docs-cn#8617
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[ran-huang]

Rest LGTM

[TomShawn]

/status LGT2

[TomShawn]

/merge

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

Commit hash: 7efe88f