statistics: add some doc for the exp feature (#9891) (#10534)

[ti-chi-bot]

This is an automated cherry-pick of #10534

This is an automated cherry-pick of #9891

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add docs for the missing part of statistics of the optimizer

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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

@TomShawn: /merge in this pull request requires 2 approval(s).

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/merge

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[TomShawn]

/merge

[ti-chi-bot]

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

Commit hash: 2129228

[TomShawn]

/merge

[TomShawn]

/verify