Don't require setting provider if there is an `ex_doc` section?

[tsloughter]

Would be nice if there wasn't a requirement to include:

{hex, [
    {doc, #{provider => ex_doc}}
]}.

In rebar.config if there is already an ex_doc section.

[starbelly]

Agreed. The only rub I can see is if you're doing both edoc and ex_doc for some reason, but could just override the default behavior with a switch in that case.