docs: add documentation for ADMIN ALTER DDL JOBS command (#19085)

[ti-chi-bot]

This is an automated cherry-pick of #19085

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

• This PR is translated from:
• Other reference link(s): Support dynamic change to DDL job configuration tidb#57229, tidb: add description for var tidb_ddl_reorg_max_write_speed #19071

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• Delete files
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[hfxsd]

/approve

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