bug#71048: The manual should have an example for treesit-language-at-point-function

[Dmitry Gutov]

https://www.gnu.org/software/emacs/manual/html_node/elisp/Multiple-Languages.html 
has code examples for multiple-language setup (with tree-sitter modes), 
and it more-or-less says that treesit-language-at-point-function should 
be set.

But there is a default behavior for when it's not set, and the code 
snippets in the second half of the page don't set it, so it seems prone 
to mistakes.

I think the examples should include a setting for 
treesit-language-at-point-function.

Inspired by https://github.com/dgutov/mmm-mode/issues/142