Clarify on_ready triggering more than twice.

On that note, fix the background_task example to use `Client.run`
instead of manual loop management. That way the task will be cleaned
up and cancelled inside `Client.run`.

examples/background_task.py

@@ -19,13 +19,5 @@ async def on_ready():
     print(client.user.id)
     print('------')
 
-loop = asyncio.get_event_loop()
-
-try:
-    loop.create_task(my_background_task())
-    loop.run_until_complete(client.login('token'))
-    loop.run_until_complete(client.connect())
-except Exception:
-    loop.run_until_complete(client.close())
-finally:
-    loop.close()
+client.loop.create_task(my_background_task())
+client.run('token')