doc: ObjectWrap requires manual cleanup on shutdown

Clarify that ObjectWrap instances are not destroyed on process or
worker shutdown and require manual destruction in that case.

doc/api/addons.md

@@ -965,6 +965,10 @@ provided by the underlying V8 JavaScript engine. They are subject to change
 or removal at any time. They are not documented by Node.js or V8, and they
 should never be used outside of testing.
 
+Note that during shutdown of the process or worker threads destructors are not
+called by the JS engine. Therefore it's in the responsibility of the user to
+monitor these objects and ensure proper destruction and avoid resource leaks.
+
 ### Factory of wrapped objects
 
 Alternatively, it is possible to use a factory pattern to avoid explicitly