imprv: enable config timestamp

Add comments.

apps/app/src/migrations/20230731075753-add_installed_date_to_config.js

@@ -21,6 +21,10 @@ module.exports = {
       const users = await User.find().limit(1).sort({ createdAt: 1 });
       const initialUserCreatedAt = users[0].createdAt;
       logger.debug('initialUserCreatedAt: ', initialUserCreatedAt);
+
+      // Set app:installed date.
+      // refs: https://mongoosejs.com/docs/6.x/docs/timestamps.html#disabling-timestamps
+      //       Read the section after "Disabling timestamps also lets you set timestamps yourself..."
       const updatedConfig = await Config.findOneAndUpdate({ _id: appInstalled._id }, { createdAt: initialUserCreatedAt }, {
         new: true,
         timestamps: false,
@@ -39,6 +43,8 @@ module.exports = {
 
     const appInstalled = await Config.findOne({ key: 'app:installed' });
     if (appInstalled != null) {
+
+      // Unset app:installed date.
       const updatedConfig = await Config.findOneAndUpdate({ _id: appInstalled._id }, { $unset: { createdAt: 1 } }, {
         new: true,
         timestamps: false,