docstring is for users to use this library itself. So please write how to use this library from the user's point of view.
Example is like this:

```
"""Handles authentication.

This class handles authentication for http requests.  The typical usecase
to use this class is as follows:

  >>> class MyHandler(AbstractAuthHandler):
  ...  @tornado.web.authenticated
  ...  def get(self):
  ...    self.write("hello, world")

  >>>  settings = {
  ...      "cookie_secret": "ADA7D03B-B889-45C7-ACB9-423DDD88A725",
  ...      "login_url": "/login",
  ...      "xsrf_cookies": True
  ...  }

  >>>  app = tornado.web.Application(
  ...  [
  ...      (r'/', LsHandler),
  ...      (r'/login', LoginHandler),
  ...      (r'/logout', LogoutHandler),
  ...  ],
  ...  **settings)

```