doc: os.uptime() temporary bug notice

Makes it clear that os.uptime() is unreliable in some virtualization
cases.

PR-URL: https://github.com/nodejs/node/pull/36503


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>