docs(#3241): add api.Decorator and Class meta with generated help, harden decorator rendering, api.decorator.UserDecorator changed to nvim_tree.api.Decorator in a non-breaking manner

docs(#3241): add api.Decorator and Class meta with generated help, harden decorator rendering, api.decorator.UserDecorator changed to nvim_tree.api.Decorator in a non-breaking manner #2440