Add Godot.SignalAwaiter class documentation and reference

**Your Godot version:**
3.2.1
**Issue description:**
There's no documentation regarding the public class SignalAwaiter.
It should also be noted that it's the object type returned by Node.ToSignal, and awaiting it consumes the object, therefore requiring to create a new SignalAwaiter every time.
**URL to the documentation page (if already existing):**
It should be cited also here(?) https://docs.godotengine.org/en/stable/getting_started/scripting/c_sharp/c_sharp_differences.html#yield