Add doc for tiflash_hash_join_version system variable

[gengliqi]

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Timeline:

• 2025-07-01 11:27:27.654636622 +0000 UTC m=+1394300.377815602: ☑️ agreed by Oreoxmt.
• 2025-07-18 08:02:38.701026972 +0000 UTC m=+2850811.424205950: ☑️ agreed by qiancai.

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