In the revised documentation there's a broken reference in Field Resolvers/Examples/Formatting a Date to "resolver factory", since the section about the resolver factory functionality of attach-resolvers is gone. My thought is that instead of that reference you could put an example of constructing a resolver function, such as:

(fn [k fmt]
  (fn [context args resolved-value]
    (->> resolved-value
         k
         (format fmt))))