Add more useful inline documentation to modules section of config

Author: adamwathan

defaultConfig.stub.js

@@ -765,6 +765,10 @@ module.exports = {
   | Here is where you control which modules are generated and what variants are
   | generated for each of those modules.
   |
+  | Currently supported variants: 'responsive', 'hover', 'focus'
+  |
+  | To disable a module completely, use `false` instead of an array.
+  |
   */
 
   modules: {