[Doc] Spark load can not be used to load data into a Primary Key table (backport #29791)

[mergify]

This is an automatic backport of pull request #29791 done by Mergify.
Cherry-pick of 9cc630a has failed:

On branch mergify/bp/branch-2.5/pr-29791
Your branch is up to date with 'origin/branch-2.5'.

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	both modified:   docs/loading/SparkLoad.md

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[mergify]

@mergify[bot]: Backport conflict, please reslove the conflict and resubmit the pr

[wanpengfei-git]

[FE Incremental Coverage Report]

😍 pass : 0 / 0 (0%)

[wanpengfei-git]

[BE Incremental Coverage Report]

😍 pass : 0 / 0 (0%)