Updated Japanese API documentation

[kojix2]

• Use code block syntax in api.ja.me

  • Added syntax highlighting to Markdown code blocks.

• Update api.ja documentation with additional details

  • I added some text to make it easier to understand.
  • This part has been changed because it differs from the actual implementation.

    ndfunc_alloc関数で ndfunc_t構造体に登録し、ndfunc_do関数で多次元ループ処理を行う。

Please review. Thank you.