doc: clarify UV_THREADPOOL_SIZE env var usage

Setting of UV_THREADPOOL_SIZE from within javascript using
process.env.UV_THREADPOOL_SIZE is not guaranteed to work as
the thread pool would have been created as part of the runtime
initialisation much before user code is run.

update doc/api/cli.md

Author: preveen-stack

doc/api/cli.md

@@ -3450,8 +3450,9 @@ reason any of these APIs takes a long time, other (seemingly unrelated) APIs
 that run in libuv's threadpool will experience degraded performance. In order to
 mitigate this issue, one potential solution is to increase the size of libuv's
 threadpool by setting the `'UV_THREADPOOL_SIZE'` environment variable to a value
-greater than `4` (its current default value). For more information, see the
-[libuv threadpool documentation][].
+greater than `4` (its current default value). However, setting this from inside 
+javascript using `process.env.UV_THREADPOOL_SIZE=size` is not guranteed to work.
+For more information, see the [libuv threadpool documentation][].
 
 ## Useful V8 options