Rename parameters to align on the word "completion"

[jonsequitur]

        // Why is this one "values" and the other two are "complete"?
        // Since the method is called "AddCompletions" I'd expect things to be called a "completion".
        public static TArgument AddCompletions<TArgument>(this TArgument argument, string[] values);
        public static TArgument AddCompletions<TArgument>(this TArgument argument, Func<CompletionContext,IEnumerable<string>> complete);
        public static TArgument AddCompletions<TArgument>(this TArgument argument, CompletionDelegate complete);