title: Flask-Restx Swagger Doc issue
body: <p>I am getting AttributeError: module 'flask_restx.api' has no attribute 'doc' when it try add additional for API when using flask restx. How this error can be fixed.</p>
  <p>api.py</p>
  <pre><code>from flask import Flask
  from flask_restx import Api, Resource
  from hello import Hello
  
  app = Flask(__name__)
  api = Api(app)
  
  api.add_resource(Hello, '/hello')
  
  if __name__ == '__main__':
      app.run(debug=True)
  </code></pre>
  <p>hello.py</p>
  <pre><code>from flask_restx import Resource, api
  
  
  @api.doc(params={'id': 'An ID'})
  class Hello(Resource):
      def get(self):
          return {
              'data': {
                  'names': ['one',
                            'two',
                            'three']
              }
          }
  </code></pre>