[Doc] Update doc about kernels and functions

[lin-hitonami]

Issue: #8387

Brief Summary

🤖 Generated by Copilot at aff59bb

This pull request updates the documentation of kernels and Taichi functions in docs/lang/articles/kernels/kernel_function.md. It introduces the concepts of Taichi real functions and multiple return values for kernels, and explains how they differ from other types of Taichi functions and kernels.

Walkthrough

🤖 Generated by Copilot at aff59bb

• Introduce and document the new feature of Taichi real functions, which allow recursive calls and multiple return statements, using the @ti.real_func decorator (link, link)
• Update the definition and terminology of Taichi functions to include both Taichi inline functions and Taichi real functions, and explain their differences (link, link)
• Emphasize the requirement of type hinting the arguments of kernels, which is different from Taichi functions (link)
• Document and demonstrate the new feature of kernels having multiple return values, using tuples as return types (link, link)

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