statistics: add some doc for the exp feature

[winoros]

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add docs for the missing part of statistics of the optimizer

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• lilin90

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[songrijie]

LGTM

[TomShawn]

@lilin90 All comments are addressed in https://github.com/pingcap/docs/pull/9891/commits/ab71359554491169b764506bee0d5097884250ff. PTAL again. Thanks!

[TomShawn]

/verify

[TomShawn]

/remove-status LGT1 /status LGT2

[TomShawn]

/merge

[ti-chi-bot]

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

Commit hash: 820992a3c8f3fe578ccf2bfb8e8263300eaefb09

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10533.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10534.