Can't find some documentation for 12.5 custom resources

I’m trying to put together an internal class about writing custom resources. I don’t see any current doc for some of the parameter validators. I had to read the code to see that the old attribute style validators still work with the parameter definitions. So, before I put together a pull request updating the doc I’d like to know if any of these validators going to be deprecated soon: equal_to, required, coerce, callbacks, respond_to, cannot_be, name_attribute, name_property, instance_variable_name.

I also think the action_class documentation should be in the custom resource documentation. The custom resources notes describe action_class but it’s not obvious a person needs to read those notes to understand 12.5 style resources. I’ll try to include something if it makes sense. Opinions?

Submitted a pull request for the validation parameters.