sys var: clarify the description of tidb_ddl_reorg_max_write_speed

[CbcWestwolf]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-03-14 04:02:19.015174769 +0000 UTC m=+501294.620703695: ☑️ agreed by hfxsd.
• 2025-03-14 04:05:46.803420408 +0000 UTC m=+501502.408949334: ☑️ agreed by tangenta.

[hfxsd]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hfxsd

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #20053.
But this PR has conflicts, please resolve them!

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #20054.
But this PR has conflicts, please resolve them!

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #20055.

[CbcWestwolf]

/cherrypick release-8.1

[ti-chi-bot]

@CbcWestwolf: new pull request created to branch release-8.1: #20540.
But this PR has conflicts, please resolve them!

Details

In response to this:

/cherrypick release-8.1

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