revise description of TSO variables

[en-jin19]

What is changed, added or deleted? (Required)

Revise a part of unclear description of TSO variables according to comments on pingcap/docs#6770.

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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

/remove-status LGT1
/status LGT2

[TomShawn]

/merge

[ti-chi-bot]

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

Commit hash: 0681a9c