No mentions that signals can't be emitted from any thread other than main

**Your Godot version:** 4.1 stable

**Issue description:**
After several hours trying to figure out why I get really weird error messages from my multi threaded code I find out in a test scenario that I cannot emit a signal from another thread than main. I haven't seen any documentation of this in any of the reasonable places (Signal Class documentation, Object.emit_signal, or the tutorial on using signals)

**URL to the documentation page (if already existing):**

[Object](https://docs.godotengine.org/en/4.1/classes/class_object.html)
[Signal](https://docs.godotengine.org/en/4.1/classes/class_signal.html)
[Using signals](https://docs.godotengine.org/en/4.1/getting_started/step_by_step/signals.html)