Fix to-base --type handler to correctly set std::optional flag

Now that we have a separate flag function, also describe why it is
optional.

src/libutil/args.hh

@@ -85,6 +85,7 @@ protected:
         Handler handler;
 
         static Flag mkHashTypeFlag(std::string && longName, HashType * ht);
+        static Flag mkHashTypeOptFlag(std::string && longName, std::optional<HashType> * oht);
     };
 
     std::map<std::string, Flag::ptr> longFlags;