doc: threadpool size, and APIs using the pool

Not knowing which APIs use libuv's threadpool can lead to surprising
performance problems. Document the APIs, and also document
UV_THREADPOOL_SIZE, which can be used to fix problems.

PR-URL: https://github.com/nodejs/node/pull/14995


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>