Multithreading tutorial doesn't conatin a method for joining threads not connected to a node

**Your Godot version:**
4.3
**Issue description:**
While the documentation clearly states that you have to join a thread that's been spawned to the main thread upon application exit, there is no example on how this is done when you have no easy access to the scene tree, like inside of a Resource or Ref-counted derived class. I've tried to find this online, but can't seem to find the information and think that is is worth having in the documentation. Is this something that can easily be done and is reasonable to add to the docs, or does this feature not exist in godot. Something along the lines of Engine.on_quit_called or something?
**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/tutorials/performance/using_multiple_threads.html