Add description and comments

test/function/handles-stringified-sourcemaps/_config.js

@@ -1,10 +1,13 @@
 module.exports = {
+	description: 'handles transforms that return stringified source maps (#377)',
+
 	options: {
 		plugins: [
 			{
 				transform: function ( code ) {
 					return {
 						code: code,
+						// just stringify an otherwise acceptable source map
 						map: JSON.stringify({ mappings: '' })
 					};
 				}
@@ -12,6 +15,7 @@ module.exports = {
 		]
 	},
 
+	// ensure source maps are generated
 	bundleOptions: {
 		sourceMap: true
 	}