C#: Document Object _init replaced with default constructor

This distinction isn't remarked at all in the documents and its not really made clear anywhere that this is specifically what should be done. Even as a C# developer I was unsure if this was desirable in Godot to use the constructor since I thought it could be called more then once.

On the side if its not actually desirable to use the default constructor for _init, it would seem there is not equal alternative, in which case that should be documented.

In either case the difference between _init behavior and default constructor behavior if any exists should also be documented.