doc: os.uptime() temporary bug notice

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

PR-URL: #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>

doc/api/os.md

@@ -368,6 +368,18 @@ changes:
 
 Returns the system uptime in number of seconds.
 
+The value returned can be inaccurate in some
+rare virtualization cases. The issue arises when the virtualized
+guest instance shares the kernel with the host system.
+Due to the fact that libuv uses a syscall that
+provides host's uptime instead of guest's
+uptime on OpenVZ, `os.uptime()` may also provide
+erroneous result.
+
+Please refer to <https://github.com/nodejs/node/issues/36244> and
+<https://github.com/libuv/libuv/issues/3068> for an exploration of
+this issue, until it is resolved by libuv.
+
 ## `os.userInfo([options])`
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