Additional docs: c# mainloop needs to be registered in the global class

(cherry picked from commit 3122b368c0)

doc/classes/MainLoop.xml

@@ -30,6 +30,7 @@
 		[csharp]
 		using Godot;
 
+		[GlobalClass]
 		public partial class CustomMainLoop : MainLoop
 		{
 		    private double _timeElapsed = 0;