statistics: update the note for sampling rate

[qiancai]

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Removed the description about the 0 results of STATS_META after importing data, because the related issue is resolved in pingcap/tidb#48562 starting from v7.6.0.

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[LGTM Timeline notifier]

Timeline:

• 2024-06-04 06:16:39.148378639 +0000 UTC m=+3361952.905514205: ☑️ agreed by lance6716.
• 2024-06-04 07:11:26.058341505 +0000 UTC m=+3365239.815477088: ☑️ agreed by Oreoxmt.

[qiancai]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.6: #17553.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.0: #17554.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #17555.