tidb: add description for var tidb_ddl_reorg_max_write_speed (#19071)

[ti-chi-bot]

This is an automated cherry-pick of #19071

What is changed, added or deleted? (Required)

add description for var tidb_ddl_reorg_max_write_speed

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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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[Oreoxmt]

/hold