Document that concatenation is enabled by default (#2505)

Author: rluba

src/content/plugins/module-concatenation-plugin.md

@@ -10,12 +10,13 @@ related:
 
 In the past, one of webpack’s trade-offs when bundling was that each module in your bundle would be wrapped in individual function closures. These wrapper functions made it slower for your JavaScript to execute in the browser. In comparison, tools like Closure Compiler and RollupJS ‘hoist’ or concatenate the scope of all your modules into one closure and allow for your code to have a faster execution time in the browser.
 
-This plugin will enable the same concatenation behavior in webpack.
+This plugin will enable the same concatenation behavior in webpack. By default this plugin is already enabled in [production `mode`](/concepts/mode/#mode-production) and disabled otherwise. If you need it in other modes, you can add it manually:
 
 ```js
 new webpack.optimize.ModuleConcatenationPlugin();
 ```
 
+
 > This concatenation behavior is called “scope hoisting.”
 >
 > Scope hoisting is specifically a feature made possible by ECMAScript Module syntax. Because of this webpack may fallback to normal bundling based on what kind of modules you are using, and [other conditions](https://medium.com/webpack/webpack-freelancing-log-book-week-5-7-4764be3266f5).