Question: Why isn't setConfig documented?

I came across `FinalForm.setConfig()` while browsing the code and it comes in very handy. I wonder why it is not present in the documentation? Is it intended to be used internally only? Simply forgot? I'd be happy to file a PR to add it, but I wanted to understand first.

P.S.: Thanks for the great lib! Been using it for a few years now (redux-form) and I am now migrating to final-form.

Start by locating `FinalForm.setConfig()` in the code and inspecting how it is used to determine whether it is public or internal. Check the existing documentation structure, then confirm that the intended status and purpose of `setConfig()` are recorded clearly if it belongs in the public API documentation.

Written by the indexing model from the issue text.