docs(configuration) Update snippets to comply with webpack v4+ (#1905)

* docs(configuration) Update snippets to comply with webpack v4+

* Expand mode for more info

Author: EugeneHlushko

src/content/concepts/configuration.md

@@ -28,14 +28,15 @@ T> The most important part to take away from this document is that there are man
 
 The following examples below describe how webpack's configuration object can be both expressive and configurable because _it is code_:
 
-## The Simplest Configuration
+## Simple Configuration
 
 **webpack.config.js**
 
 ```javascript
 var path = require('path');
 
 module.exports = {
+  mode: 'development',
   entry: './foo.js',
   output: {
     path: path.resolve(__dirname, 'dist'),

src/content/configuration/configuration-languages.md

@@ -30,6 +30,7 @@ import path from 'path';
 import webpack from 'webpack';
 
 const config: webpack.Configuration = {
+  mode: 'production',
   entry: './foo.js',
   output: {
     path: path.resolve(__dirname, 'dist'),
@@ -103,6 +104,7 @@ webpack = require('webpack')
 path = require('path')
 
 config =
+  mode: 'production'
   entry: './path/to/my/entry/file.js'
   output:
     path: path.resolve(__dirname, 'dist')
@@ -152,7 +154,7 @@ const CustomPlugin = config => ({
 });
 
 export default (
-  <webpack target="web" watch>
+  <webpack target="web" watch mode="production">
     <entry path="src/index.js" />
     <resolve>
       <alias {...{

src/content/configuration/configuration-types.md

@@ -24,6 +24,7 @@ One option is to export a function from your webpack config instead of exporting
 -module.exports = {
 +module.exports = function(env, argv) {
 +  return {
++    mode: env.production ? 'production' : 'development',
 +    devtool: env.production ? 'source-maps' : 'eval',
      plugins: [
        new webpack.optimize.UglifyJsPlugin({
@@ -64,11 +65,13 @@ module.exports = [{
     libraryTarget: 'amd'
   },
   entry: './app.js',
+  mode: 'production',
 }, {
   output: {
     filename: './dist-commonjs.js',
     libraryTarget: 'commonjs'
   },
   entry: './app.js',
+  mode: 'production',
 }]
 ```

src/content/configuration/index.md

@@ -29,6 +29,13 @@ __webpack.config.js__
 const path = require('path');
 
 module.exports = {
+  <details><summary>[mode](/concepts/mode): "production", // "production" | "development" | "none"</summary>
+  [mode](/concepts/mode): "production", // enable many optimizations for production builds
+  [mode](/concepts/mode): "development", // enabled useful tools for development
+  [mode](/concepts/mode): "none", // no defaults
+  </details>
+  // Chosen mode tells webpack to use its built-in optimizations accordingly.
+
   <details><summary>[entry](/configuration/entry-context#entry): "./app/entry", // string | object | array</summary>
   [entry](/configuration/entry-context#entry): ["./app/entry1", "./app/entry2"],
   [entry](/configuration/entry-context#entry): {