Add doc for tiflash_hash_join_version system variable

[gengliqi]

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[lilin90]

/gemini review

[gemini-code-assist]

Code Review

The pull request adds documentation for the tiflash_hash_join_version system variable. The documentation includes a warning about the experimental nature of the feature and notes on its usage and limitations. Overall, the changes are clear and well-organized.

Merge Readiness

The pull request appears to be ready for merging, as it provides clear and concise documentation for a new system variable. There are no critical or high severity issues identified. I am unable to approve this pull request, and recommend that others review and approve this code before merging.