system-variables: update description of datadir

[hfxsd]

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as tittle

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By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v7.1 (TiDB 7.1 versions)
• v7.0 (TiDB 7.0 versions)
• v6.6 (TiDB 6.6 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

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• Delete files
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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• ran-huang

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[hfxsd]

/merge

[ti-chi-bot]

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

Commit hash: 246be12

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.0: #13752.

[ti-chi-bot]

In response to a cherrypick label: cannot checkout release-7.1: error checking out release-7.1: exit status 1. output: error: pathspec 'release-7.1' did not match any file(s) known to git

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #13753.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.6: #13754.