add doc for hash_join_v2

[windtalker]

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Timeline:

• 2024-09-29 06:46:25.303117592 +0000 UTC m=+164540.723330624: ☑️ agreed by Oreoxmt.
• 2024-09-29 07:37:25.860777535 +0000 UTC m=+167601.280990544: ☑️ agreed by qiancai.

[qiancai]

/approve

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