Clarify documentation related to ressource cleanup

[netixx]

Using aionetbox as suggested in the readme, I am having trouble at the end of my program.

If I use the await netbox_client.close() method on the client, I get an Exception ignored in: <function AIONetbox.__del__ at 0x7fa41f96b7a0> and sys:1: RuntimeWarning: coroutine 'ClientSession.close' was never awaited
If I don't use it, I get a ERROR:asyncio:Unclosed connector.

What is the proper way to clean the AIONetbox object ressources ?