doc: os.uptime() temporary bug notice

Checklist

• documentation is changed or added
• commit message follows commit guidelines

This commit makes it clear that os.uptime() is currently unreliable in some virtualization cases, until https://github.com/libuv/libuv/issues/3068 is resolved (and thus https://github.com/nodejs/node/issues/36244). At least one member of Node.js (@benjamingr) in the aforementioned issue appears to be in favour of at least a temporary notice, until the issue is resolved by libuv.

However, since similar issues may reappear in the future with the evolution of virtualization technology, I believe it is prudent to leave a permanent notice on os.uptime() doc, so perhaps it is also worth to consider.