More explicitly document backend deprecation

[reaperhulk]

We have previously made it so backend isn't a required argument, but we've referred to them as optional. We should update our docs to note that backends are not only not required, they're not recommended. Since a huge number of callers pass backends these functions will all continue to take the argument for the foreseeable future (many years!), but it's probably time to officially deprecate backend.

As part of this do we also want to remove the backend section of our docs?

[alex]

I'd go so far as to say we should delete the docs for them!