This simply makes no sense in a Ruby idiom (and I'm not really sure this is the best way to think of it in Java either -- I can make an API public and well-documented with or without interfaces.)

I've used dozens of Ruby gems, plugins and classes over the years that have well-documented shared APIs and none of them (obviously) have had interfaces. Interfaces are good in certain environments for hiding implementation details where class-level contracts are enforced, but in Ruby this would provide no benefit at all.

Think of it this way: since Ruby has no class-level contracts, to some extent everything is already an interface. Everything is abstract. Every Ruby object has the opportunity to re-implement functionality in an abstract way.