I see
"
Using threadpool scheduler

On Unix, add thread_handling=pool-of-threads to the my.cnf config file.

On Windows, the default for thread_handling is already preset to pool-of-threads, so nothing needs to be done. If you want to use one thread per connection add thread_handling=one-thread-per-connection to the my.cnf config file."

in the docs. This is how to enable it.

Oh yes, you are right! I completely read over that.

Perhaps a list there with the possible options and there defaults is useful?

Perhaps.
Yet, the variables also have a documentation elsewhere place https://mariadb.com/kb/en/mariadb/server-system-variables/#thread_handling
Maybe it is possible to highlight important stuff somehow, that it is not easy to overlook?

One problem is that the variables, which are described in full with defaults etc on the variables page, such as https://mariadb.com/kb/en/mariadb/server-system-variables/#thread_handling etc, are only partially described on the thread pool page at https://mariadb.com/kb/en/mariadb/thread-pool-in-mariadb/#threadpool-server-variables, and not linked.

I will create a new threadpool variables page (reducing the size of the main variables page, which is huge), and then prominently link to this on the threadpool description page.