doc: add UV_THREADPOOL_SIZE link definition

Currently is rendered as "see the [UV_THREADPOOL_SIZE][]
documentation", since there was no link definition in the file.

PR-URL: https://github.com/nodejs/node/pull/16193


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>