tidb: add description for var tidb_ddl_reorg_max_write_speed

[CbcWestwolf]

What is changed, added or deleted? (Required)

add description for var tidb_ddl_reorg_max_write_speed

Which TiDB version(s) do your changes apply to? (Required)

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

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
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• v8.3 (TiDB 8.3 versions)
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• v5.3 (TiDB 5.3 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): ddl: introduce a new system variable to control the store-write-bwlimit when ingesting tidb#57145

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-chi-bot]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-11-14 07:34:29.249435206 +0000 UTC m=+514431.440304187: ☑️ agreed by tangenta.
• 2024-11-18 05:53:12.260242456 +0000 UTC m=+853954.451111452: ☑️ agreed by qiancai.

[Oreoxmt]

Rest LGTM