Improve explanatory comment

Author: jdreesen

lib/config/parse-runtime.js

@@ -87,15 +87,15 @@ module.exports = function(argv, cwd) {
 
     const partialConfig = babel.loadPartialConfig({
         /*
-         * This is a small mystery. Even if we set the cwd & root
-         * options, deep in babel, if the filename option is not
-         * set, then it doesn't see the "cwd" directory as a valid
-         * directory where it should look for the .babelrc file.
-         * The fact that this is set to webpack.config.js is not
-         * significant at all - you could even invent a filename.
-         * However, as I'm not sure the side effects (the filename
-         * option is documented as "for error messages"), we're
-         * setting it to a realistic filename.
+         * There are two types of babel configuration:
+         * - project-wide configuration in babel.config.* files
+         * - file-relative configuration in .babelrc.* files
+         *   or package.json files with a "babel" key
+         *
+         * To detect the file-relative configuration we need
+         * to set the following values. The filename is needed
+         * for Babel as an example so that it knows where it
+         * needs to search the relative config for.
          */
         root: cwd,
         cwd: cwd,