Add information about RefCounted and C# Garbage Collector

**Your Godot version:**
4.4.1
**Issue description:**
Currently, the documentation for the RefCounted class says that RefCounted instances in a cyclic reference will not be freed correctly, but does not clarify that this is not an issue when using C# alone. There should be a brief mention of this.
**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/latest/classes/class_refcounted.html