There is no add_resource in Api

[paulzhn]

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In the minimal example of doc:

from flask_restx import Resource, Api

app = Flask(__name__)
api = Api(app)

@api.route('/hello')
class HelloWorld(Resource):
    def get(self):
        return {'hello': 'world'}

if __name__ == '__main__':
    app.run(debug=True)

it invoked the add_resource() by api directly; however, it seems like it only exists in Namespace. Is the doc wrong?

Additional context
flask-restx==0.3.0