doc: clarify that ObjectWrap requires manual cleanup on shutdown

Clarify that ObjectWrap instances are not destroyed on process or
worker shutdown and require manual destruction to avoid resource
leaks.

PR-URL: https://github.com/nodejs/node/pull/40074
Fixes: https://github.com/nodejs/node/issues/38816


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>