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: #16193
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

doc/api/zlib.md

@@ -711,5 +711,6 @@ Decompress a chunk of data with [Unzip][].
[Inflate]: #zlib_class_zlib_inflate
[Memory Usage Tuning]: #zlib_memory_usage_tuning
[Unzip]: #zlib_class_zlib_unzip
[`UV_THREADPOOL_SIZE`]: cli.html#cli_uv_threadpool_size_size
[options]: #zlib_class_options
[zlib documentation]: http://zlib.net/manual.html#Constants