system-variables: add doc for tidb_max_dist_task_nodes

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• Other reference link(s): dxf: add tidb_max_dist_task_nodes to specify max node count tidb#58937, sysvar: add validation check for tidb_max_dist_task_nodes tidb#59645

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[Frank945946]

这里建议补充自动计算节点数的算法，便于用户自己算

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