cleared up confusion for from_envvar in docs

This attempts to clear up confusion caused in the tutorial using `.from_envvar()`.  I don't think it is perfect but it might be a start.  

The issue presented in  issue #783 was that someone might try to replace the `.from_object()` line instead of just appending the `from_envvar()`.  While, when I first read the tutorial, I didn't interpret it that way, I can see where the confusion could come from.    

How this PR attempts to fix this issue if by taking out the language about removing the `.from_object` method and just mentioning that you could use them alongside one another to create a robust setup. 

Thoughts welcome!