doc: add parallelism note to os.cpus()

With the introduction of os.availableParallelism(), users should
no longer rely on os.cpus().length to determine the amount of
available parallelism. This commit adds a note to the os.cpus()
docs.

PR-URL: https://github.com/nodejs/node/pull/45895
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/os.md

@@ -140,6 +140,10 @@ The properties included on each object include:
 `nice` values are POSIX-only. On Windows, the `nice` values of all processors
 are always 0.
 
+`os.cpus().length` should not be used to calculate the amount of parallelism
+available to an application. Use
+[`os.availableParallelism()`](#osavailableparallelism) for this purpose.
+
 ## `os.devNull`
 
 <!-- YAML