Refinement function return types are not documented

[arturmuller]

Hi — I noticed that the docs don't mention the ability to return strings, Failures, etc... from refinement functions to customize error messages. Is this intentional (ie. is this just for internal use?) or just an omission?

I am happy to add it if there's interest.

[ianstormtaylor]

@arturmuller would love a PR! Good call.