[Doc] Update the doc of real functions

[lin-hitonami]

Issue: #

Brief Summary

🤖[deprecated] Generated by Copilot at 32b8bfa

Add a new section on Taichi real functions and update the existing section on real function arguments and return values in docs/lang/articles/kernels/kernel_function.md. Fix the section title and warn about a possible bug on older NVIDIA GPUs.

Walkthrough

🤖[deprecated] Generated by Copilot at 32b8bfa

• Add a new section on Taichi real functions, explaining their concept, usage, and benefits (link)
• Update the section on Taichi real function arguments and return values, showing the new syntax and semantics for passing scalars, vectors, and matrices (link)
• Add a warning about a possible bug when passing scalars by reference on older NVIDIA GPUs, and suggest a workaround (link)
• Add an example of using recursion in a real function, demonstrating its power and flexibility (link)
• Modify the section title to match the style of the other sections, using "Kernel Functions" instead of "Kernel Function" (link)

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